PRINCIPLES OF PLANT PATHOLOGY
PATH 271 (1+1)
Prepared By
DR. P. KISHORE VARMA,
ASSISTANT PROFESSOR,
DEPARTMENT OF PLANT PATHOLOGY
AGRICULTURAL COLLEGE,
ASWARAOPET 507 301
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LECTURE 1
INTRODUCTION TO PLANT PATHOLOGY
Why Plant Pathology?
Plants are essential for maintenance of life. Plants not only sustain the man and
animals, they are also the source of food for multitudes of micro-organisms living in the
ecosystem. Thus, while man has been able to subjugate plants and animals for his own
use, the competing micro-organisms still defy his efforts and claim a major share of
resources which man would like to use for himself. It is in this context that the need for
fighting the competing micro-organisms and other agencies that lack loss of productivity
has been felt. The attack on plants by these micro-organisms changed the appearance and
productivity of the crop and this observed change was called a disease. Plant diseases
have been considered as stubborn barriers to the rapid progress of food production.
We call a plant healthy only so long as it continues to perform all its normal
physiological activities and give the expected yield according to its genetic potentiality.
Physiological activities of a healthy plant
1. Normal cell division, differentiation and development.
2. Uptake of water and nutrients from the soil.
3. Synthesis of food from sunlight by photosynthesis.
4. Translocation of water and food to the sites of necessity through xylem and
phloem.
5. Metabolism of synthesized material
6. Reproduction
A diseased plant fails to perform one or more of these functions. The effect of a disease
on functioning of an organ depends on which cells or tissues were first attacked by the
pathogen.
For example, rotting of root tissues will affect the absorption of water and minerals from
soil and if vascular tissues have been affected, the translocation of water and
photosynthetates will be stopped or reduced. If leaf tissues are attacked by a pathogen,
photosynthesis is affected and plant suffers from deficiency of carbohydrates essential for
supplying energy for other activities. Thus, disease can be defined as malfunctioning
process that is caused by continuous irritation by a pathogen (Dimond, 1959).
Definitions:
1. Disease is a malfunctioning process that is caused by continuous irritation which
results in some suffering producing symptoms (American Phytopathological society
& British Mycological society).
2. Disease is an alteration in one or more of the ordered sequential series of
physiological processes culminating in a loss of coordination of energy utilization in a
plant as a result of continuous irritation from the presence or absence of some agent
or factor.
3. Disease: Any malfunctioning of host cells and tissues that result from continuous
irritation by a pathogenic agent or environmental factor and leads to development of
symptoms (G.N.Agrios, 1997).
Pathogens bring about these irritating processes through different but inter-related
pathways
1. by utilizing the host cell contents,
2. by causing death of cells or by interfering with their metabolic activities through
their enzymes, toxins and growth regulators,
3. by weakening of tissues due to continuous loss of nutrients, and
4. by interfering with translocation of food, minerals and water.
OBJECTIVES OF PLANT PATHOLOGY:
The science of plant pathology has four main objectives:
1. to study the living, non-living and environmental causes of plant diseases,
2. to study the mechanisms of disease development by pathogens,
3. to study the interactions between the plants and the pathogen, and
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4. to develop the methods of controlling the diseases and reducing the losses caused
by them.
HISTORY OF PLANT PATHOLOGY:
Ancient period:
A literature of European and vedic eras will give us some information on the plant
diseases and their control measures. Greek philosopher Theophrastus recorded some
observations on the plant diseases in his book enquiry into plants. His experiences were
mostly based on imagination and observation but not on experimentation. He had
mentioned that plants of different groups have different diseases which were autonomous
or spontaneous, i.e., no external cause was associated.
In India, the information on plant diseases is available in ancient literature such as
rigveda, atharveda (1500-500BC), arthasashtra of Kautilya (321-186 BC), Sushruta
sanhita (200-500AD), Vishnupuran (500AD), Agnipuran (500-700AD), Vishnu
dharmottar (500-700AD), etc. In Rigveda, not only the classification of plant diseases has
been given but the germ theory of disease was also advocated.
Vriksha ayurveda by Surpal in ancient India is the first book in which lot of
information on plant diseases is available. In this book, plant diseases were categorized
into two groups, internal (probably physiological diseases) and external (probably
infectious diseases). External diseases were supposed to be due to attack of
microorganisms and insects. In this book, a mention of treatments for different diseases
caused by different agencies was prescribed which were based on superstition as well as
scientific observation. Hygiene, tree surgery, protective covering with pastes and special
culture of plants are practices which are still recommended. In chemical treatments, use
of honey, ghee, milk, barley flour, pastes made from herbs, plant extracts, etc., were
recommended. For the control of root diseases, oilcakes of mahuva, mustard, sesame,
castor, etc., were used.
Symptoms of plant diseases such as rust, downy mildew, powdery mildew and blight are
often mentioned in the bible, Shakesphere’s poems and dramas of other Christian
literature. In Jataka of Buddhism, Raghuvansh of Kalidas there was also a mention about
different symptoms of plant diseases.
In Europe and other western countries, after the time of Theophrastus (about 286 BC) no
definite opinion could be formed about plant disease for the next 2000 years. In ancient
period, the plant diseases were attributed to many causes which include divine power,
religious belief, superstition, effect of stars and moon, bad wind and wrath of god, etc.
PRE-MODERN PERIOD
1) PIER ANTONIO MICHELLI (Italian):
¾ He was an Italian botanist.
¾ He was the founder and father of Mycology.
¾ He was the first person who observed fungal spores for the first time and
conducted many spore germination studies (by growing fungus organisms on
freshly cut pieces of melons and pears).
¾ He was the first person who observed Cystidia on the lamellar edge or hymenial
layer of Agaricales.
¾ In 1729 he published a book “Nova Plantarum Genera” in which he gave
descriptions about 1900 species in Latin out of which 900 were fungi. The
important genera are Aspergillus niger, Botrytis sps., Polyporus sps. etc.
2) TILLET (French)
¾ In 1755, he published a paper on bunt or stinking smut of wheat
¾ By well planned experiments he proved that wheat seeds that contained black
powder on their surface produced more diseased plants than clean seeds.
¾ He emphasized that bunt was an infectious disease and it was closely related with
fungus. However, he believed that the disease was caused by some toxin produced
by the black powder. He did not know that the black powder contained the spore
mass of the fungus.
¾ He reported that the chemical treatment of seeds with common salt and lime
inhibited the contagious activity.
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MODERN PERIOD
1) BENEDICT PREVOST (French)
¾ He proved that diseases are caused by micro-organisms
¾ He studied wheat bunt disease for about 10 years and in 1807, he published his
findings in the paper “memoir on the immediate cause of bunt or smut of wheat
and of several other diseases of plants and on preventives of bunt”
¾ He proved that the bunt of wheat was caused by the fungus Tilletia caries
¾ Studied and observed the germination of bunt species. He confirmed the findings
of Tillet by mixing the spores of fungus with clean seeds.
¾ Discovered the life cycle of bunt fungus
¾ He showed that the solution containing copper sulphate prevented the germination
of bunt spores and can be used for control of bunt diseases.
¾ He mentioned the fungicidal and fungistatic properties of chemical treatments
2) CHRISTIAN HENDRICK PERSOON (1761-1831):
¾ Persoon first published observations Mycologicae.
¾ In 1801, he published “Synopsis methodica fungorum” for nomenclature of
Ustilaginales, Uredinales and Gasteromycetes.
¾ He also published Mycologica Europica in 1822.
¾ He gave the name to rust pathogen of wheat as Puccinia graminis.
3) ELIAS MAGNUS FRIES (1821):
¾ He published three volumes of “Systema Mycologium” for nomenclature of
hymenomycetes.
¾ Person and fries first time introduced binomial system of nomenclature to classify
the fungal organisms.
During 1830-1845, when late blight of potato was fast spreading in England, Ireland and
continental Europe, there was no one opinion among the scientists about the diseasefungus relationship.
1) ANTON De BARY (Germany):
¾ He was the father and founder of modern Mycology.
¾ He was the founder of modern experimental plant pathology
¾ In 1863, he studied the epidemics of late blight and renamed the casual organism
as Phytophthora infestans.
¾ He discovered heteroecious nature of rust fungi (1865).
¾ He gave detailed account on life cycles of downy mildew genera.
¾ He studied about vegetable rotting fungi and damping off fungi.
¾ He wrote a book named “Morphology and Physiology of fungi, lichens and
Myxomycetes” (1866).
¾ He reported the role of enzymes and toxins in tissue disintegration caused by
Sclerotinia sclerotiorum
Students of De Bary:
1. Marshal Ward (UK)
2. M.S. Woronin (USSR)
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3. Farlow
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4. Millardet
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Studied coffee rusts and its epidemics
Studied about life cycle of club root fungi,
i.e, Plasmodiophora brassica
Fungi and bibliography. He established
Farlow cryptogamic herbarium. Farlow,
first introduced independent course of plant
pathology at Harward University.
Discovered Bordeaux mixture for the
control of downy mildew of grapevine
Oscar Brefeld, a colleague of De Bary (Germany) -Pioneer in pure culture techniques
2) E. J. Butler (Edwin John Butler):
¾ He was the father of modern plant pathology and father of Indian Mycology.
¾ He worked at IARI for 20 years from 1901 to 1920.
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¾ He was the founder and first director of imperial Mycological institute, Kew,
England (1920-35).
¾ Monograph: Pythiaceous and allied fungi.
¾ Books: a) Fungi and Disease in Plants (1918)
b) Fungi in India (with B.R.Bisby) and
c) Plant Pathology (with S.G.Jones).
3) E.C. STAKMAN
¾ He studies the variability in rust fungus. Contributed valuable information on
physiological races of pathogen
¾ He concluded that due to continuous evolution of races and biotypes in the species
of the rust fungus its pathogenic capability goes on changing and as a result the
resistant capability of the host also changes.
4) T. J. BURRUILL (USA): He proved for the first time that fire blight of apple and
pear was caused by a bacterium (now known as Erwinia amylovora)
5) E.F.SMITH (U.S.A)
¾ He gave the final proof of the fact that bacteria could be incitants of plant
diseases.
¾ He also worked on the bacterial wilt of cucurbits and crown gall disease. He is
also called as "Father of Phytobacteriology".
¾ In 1981, he demonstrated for the first time that budding or grafting could be
another method of transmission of plant viruses.
¾ He showed the contagious nature of peach yellows.
6) DOI AND ISHIE (JAPANESE)
¾ They found that mycoplasma like organisms (MLO) could be responsible for the
disease of the yellows type.
¾ Doi observed that MLO's are constantly present in phloem while Ishie observed
MLO's temporarily disappeared when the plants are treated with tetracycline
antibodies.
7) BEIJERINCK (Dutch)
¾ Founder of virology
¾ He proved that the virus inciting tobacco mosaic is not a living microorganism.
¾ He believed it to be contagium vivum fluidum (infectious living fluid)
8) W.H.STANLEY
¾ In 1935, he proved that viruses can be crystallised. He got Nobel Prize.
¾ He treated the sap from diseased leaves of tobacco with ammonium sulphate and
obtained a crystalline protein which, when placed on healthy tobacco leaves,
could reproduce the disease.
¾ He finally proved that viruses are not living micro-organisms because no living
form can be chemically treated and crystallized and still remain viable.
9) BAWDEN F.E. and PIRIE (Britain): They found that the crystalline nature of the
virus contains nucleic acid and protein.
10) DIENER and RAYMER discovered the potato spindle tuber was caused by small
naked ssRNA which he called viroid.
INDIAN SCIENTISTS
1) B.B MUNDKUR:
¾ He worked on the control of cotton wilt diseases.
¾ He is responsible for the identification and classification of large number of
Indian smut fungi
¾ He started Indian Phytopathological Society in 1948 and published a journal
Indian Phytopathology.
¾ His book – Fungi and Plant diseases.
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2) J.F.DASTUR:
¾ First Indian plant pathologist who was credited for his detailed studies on fungi
and plant diseases.
¾ He studied the characters of Phytophthora and Phytophthora diseases of potato
and castor.
¾ He established Phytophthora parasitica from castor.
3) K.C. MEHTA – Life cycle of cereal rusts in India
4) T.S. SADASIVAN
¾ Started the studies on bio-chemistry of host-parasite relationship at University of
Madras
¾ Contributed to the concept of vivotoxins
¾ Studied on mechanism of wilting in cotton by Fusarium vasinfectum. The
production of fusaric acid by this fungus outside the host was demonstrated.
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LECTURE 2
TERMS AND CONCEPTS USED IN PLANT PATHOLOGY
Disease: Any malfunctioning of host cells and tissues that result from continuous
irritation by a pathogenic agent or environmental factor and leads to development of
symptoms (G.N.Agrios, 1997).
Disorder: Non-infectious plant diseases due to abiotic causes such as adverse soil and
environmental conditions are termed disorders. The common characteristic of noninfectious diseases of plants is that they are caused by the lack or excess of something
(temperature, soil moisture, soil nutrients, light, air and soil pollutants, air humidity, soil
structure and pH) that supports life. Non-infectious plant diseases occur in the absence of
pathogens, and cannot, therefore, be transmitted from diseased to healthy plants.
Pathogen: An entity, usually a micro-organism that can incite disease. In a literal sense a
pathogen is any agent that causes pathos (ailment, suffering) or damage. However, the
term is generally used to denote living organisms (Fungi, bacteria, MLO’s, nematodes
etc.,) and viruses but not nutritional deficiencies.
Parasite: Organisms which derive the materials they need for growth from living plants
(host or suscept) are called parasites.
Pathogenicity is the ability of the pathogen to cause disease
Pathogenesis is the chain of events that lead to development of disease in the host (or)
sequence of progress in disease development from the initial contact between the
pathogen and its host to the completion of the syndrome
Sign: The pathogen or its parts or products seen on a host plant.
Symptom: The external or internal reactions or alterations of a plant as a result of a
disease.
Syndrome: The set of varying symptoms characterizing a disease are collectively called
a syndrome.
Biotroph: An organism that can live and multiply only on another living organism. They
always obtain their food from living tissues on which they complete their life cycle.
Ex: Rust, smut and powdery mildew fungi.
Hemibiotroph (Facultative Saprophyte): The parasites which attack living tissues in
the same way as biotrophs but will continue to grow and reproduce after the tissue is dead
called as facultative saprophytes.
Perthotrophs or perthophytes (Necrotroph): A parasite is a necrotroph when it kills
the host tissues in advance of penetration and then lives saprophytically
Ex: Sclerotium rolfsii.
Inoculum: It is the part of the pathogen which on contact with susceptible host plant
causes infection (or) the infective propagules which on coming in contact with the host
plant causes an infection are known as inoculum
Inoculum potential: The energy of growth of a parasite available for infection of a host
at the surface of the host organ to be infected (or) The resultant of the action of
environment, the vigour of the pathogen to establish an infection, the susceptibility of the
host and the amount of inoculum present
Incubation period: The period of time (or time lapse) between penetration of a host by a
pathogen and the first appearance of symptoms on the host. It varies with pathogens,
hosts and environmental conditions.
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Predisposition: It is the action of set of environments, prior to penetration and infection,
which makes the plant vulnerable to attack by the pathogen. It is related to the effect of
environments on the host, not on the pathogen, just before actual penetration occurs
Hypersensitivity: Excessive sensitivity of plant tissues to certain pathogens. Affected
cells are killed quickly, blocking the advance of obligate parasites.
Infection is the establishment of parasitic relationship between two organisms, following
entry or penetration (or) the establishment of a parasite within a host plant.
Systemic infection: The growth of pathogen from the point of entry to varying extents
without showing adverse effect on tissues through which it passes.
Epidemic or Epiphytotic disease: A disease usually occurs widely but periodically in a
destructive form is referred as epidemic or Epiphytotic disease.
Ex: Late blight of potato – Irish famine (1845)
Endemic: Constantly present in a moderate to severe form and is confined to a particular
country or district.
Ex: Club root of cabbage in Nilgiris
Black wart of potato – Synchytrium endobioticum
Onion smut – Urocystis cepulae
Sporadic disease: Occur at very irregular intervals and locations and in relatively fewer
instances. Ex: Udbatta disease of rice, Angular leaf spot of cucumber –
Pseudomonas lachrymans
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LECTURE 3
SURVIVAL OF PLANT PATHOGENS
The means of survival are the first link in infection chain or disease cycle. The initial
infection that occurs from the sources of pathogen survival (Infected host as a reservoir of
inoculum, saprophytic survival outside the host or dormant spores and other structures in
or on the host or outside the host) in the crop is primary infection and the propagules that
cause this infection are called primary inoculum. After initiation of the disease in the
crop, the spores or other structures of the pathogen are sources of secondary inoculum
and cause secondary infection, thereby spreading the disease in the field.
Ex: The fungus (Phytophthora infestans) causing late blight of potato survives in seed
tubers or in soil. Infected tubers bring the primary infection in the field while primary
inoculum (PI) present in soil causes primary infection of the crop from healthy seed. The
PI may also be brought by wind from neighboring fields or long distances. Then the
fungus produces spores on leaves. These spores are dispersed by wind and water and
reach healthy plant surfaces to cause new infections. This is secondary infection. The
primary infection initiates the disease and secondary infection spreads the disease.
SOURCES OF SURVIVAL OF PATHOGENS:
1) Infected host as reservoir of inoculum (or) survival in vital association with living
plants.
2) Survival as saprophytes outside the host.
3) Survival by means of specialized resting structures in or on the host or outside the host.
4) Survival in association with insects, nematodes and fungi.
1) Infected host as reservoir of inoculum:
The infected host serving as reservoir of active inoculum is grouped into
a) Seed: Seed may be externally or internally infected by plant pathogens during the
course of development and maturation in fruit or pod. Most seed borne pathogens
survive as long as seed remains viable.
Ex 1: The pathogen of loose smut of wheat, Ustilago nuda tritici, enters the stigma
and style and infects the young seed, in which it survives as mycelium.
Ex 2: Pseudomonas syringae pv. tomato has been shown to survive in dried tomato
seed for 20 years.
b) Collateral hosts / Alternative hosts (wild hosts of same families): Collateral hosts
are those which are susceptible to the plant pathogens of crop plants and provide
adequate facilities for their growth and reproduction of these pathogens during offseason. Weeds which survive and live during non-cropping season provide for the
continuous growth and multiplication of the pathogen. Thus the weed hosts help to
bridge the gap between two crop seasons.
Ex: The fungal pathogen for blast disease of rice, Pyricularia grisea (Teleomorph:
Magnaporthe grisea) can infect the grass weeds like Brachiaria mutica, Dinebra
retroflexa, Leersia hexandra, Panicum repens etc., and survive during off-season of
rice crop. As soon as a fresh rice crop is raised, the conidia (inoculum) liberated from
the weed host disseminated by wind infects the fresh rice crop.
c) Alternate hosts (Wild hosts of other families): The role of alternate hosts is not as
important as of collateral hosts. However, when a pathogen has very wide host range
(as Sclerotium rolfsii, Rhizoctonia solani, Fusarium moniliforme etc.) and is tolerant
to wide range of weather conditions the alternate hosts become very important source
of survival of the pathogen. These alternate hosts are very important for the
completion of the life cycle of heteroecious rust pathogens.
For example in temperate regions the alternate host of Puccinia graminis tritici (black
or stem rust pathogen of wheat), the barberry bush (Berberis vulgaris) grows side by
side with the cultivated host. In such areas this wild host belonging to a different
family is important for survival of the fungus.
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d) Self sown crops: Self sown crops, voluntary crops and early sown crops are
reservoirs of many plant pathogens. Ex: Self sown rice plants harbour the pathogen
(Rice tungro virus) as well as vector (Nephottetix virescens).
e) Ratoon crops: Sometimes ratoon crops also harbour the plant pathogens.
Ex: Sugarcane mosaic.
f) Survival by latent infection: Latent infection refers to the conditions in which the
plant pathogens may survive for a long time in plant tissue without development of
visual symptoms. Ex: Xylella fastidiosa, the causal agent of pierce’s disease of
grapevine infect different weeds without developing visible symptoms.
2) Saprophytic survival outside the host:
The ability to live saprophytically enables many plant pathogens to survive in the
absence of growing susceptible plants. Saprophytic survival usually occurs in or on
the soil. Waksman (1971) distinguished between soil inhabitants and soil invaders;
the former comprise the basic fungal flora of the soil, whereas the later are short lived
exotics.
In the absence of the cultivated host plant, fungi are capable of surviving as
saprophytes and can be studied under three categories:
1) Soil inhabitants: Those organisms which survive indefinitely in the soil as
saprophytes in the absence of the host plant. Ex: Species of Pythium, Rhizoctonia
and Sclerotium
2) Root inhabitants: These are more specialized parasites that survive in soils in
close association with their hosts. The active saprophytic phase remains as long as
the host tissue in which they are living as parasites is not completely decomposed.
Ex: Species of Fusarium, Verticillium (vascular wilt causing fungi) and root rot of
cotton (Phymatotrichum omnivorum)
3) Rhizosphere colonizers: Those organisms which colonize the dead substrates in
the root region and continue to live like that for a longer period which are more
tolerant to soil antagonism. Ex: Leaf mold in tomato: Cladosporium fulvum
Differentiate Soil inhabitants and soil invaders:
Soil inhabitants
Soil invaders / Root inhabiting fungi
1. These are unspecialized parasites
with a wide host range that are able
to survive indefinitely in the soil as
saprophytes.
1. These are more specialized parasites that
survive in soils in close association with
their hosts.
2. Soil inhabitants include obligate
saprophytes and facultative parasites
they are exo-pathogens
2. Soil invaders include facultative
saprophytes which are endo-pathogens
(root infecting fungi).
3. Soil and plant debris serve as media
for their saprophytic survival.
3. The active saprophytic phase remains as
long as the host tissue in which they were
living as parasites is not completely decomposed.
4. They have high competitive saprophytic survival ability.
4. They have low competitive saprophytic
survival ability.
5. Species of Pythium, Rhizoctonia,
5. Most plant pathogenic fungi and bacteria
Sclerotium, etc., survive as soil inhabiare soil invaders. Many
tants
for considerable length of time in
vascular wilt causing species of Fusarium,
absence of the host.
Verticillium, etc., are soil invaders.
3) Survival as dormant spores or specialized resting structures:
Plant viruses have no resting stage and are transmitted through a continuous infection
chain.
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Phytopathogenic bacteria: The plant bacteria also do not produce resting spores or
similar structures. They continuously live in their active parasitic stage in the living host
or as active saprophytes on dead plant debris.
Nematodes: They survive in the form of active parasitic phase on a living host and also
survive through dormant structures, i.e., eggs, cysts, galls, formed in host tissues. These
structures may be present in soil or in seed lots
Phanerogamic parasites: They survive in dormant state for many years through seeds.
Ex; Seeds of Orobanchae survive in soil for more than 7 years.
Among plant pathogens, fungi are the only organisms that produce spores, analogous to
eggs of nematodes, and other resting structures for their inactive survival. These dormant
structures of survival can be classified in the following categories.
1) Soil borne fungi:
a) Dormant spores {Conidia (Peach leaf curl pathogen, Taphrina deformans),
Chlamydospores (Wilt pathogen, Fusarium sp.), oospores (Downy mildew fungi),
perithecia (Apple scab pathogen, Venturia inaequalis) etc.}.
Oospore
Chlamydospores
Perithecium
b) Other dormant structures such as thickened hypha, sclerotia (Cottony rot fungus,
Sclerotinia sclerotiorum), microsclerotia (Verticillium), Rhizomorphs (Armillaria
mellea), etc.
Thickened hyphae
Sclerotia
Rhizomorphs
Microsclerotia
c) Factors affecting the survival of pathogen in the soil are a) physical factors (high
temperature, irradiation, dessication and anaerobiosis), b) chemical factors
(antibiotics, antagonistic chemicals produced by other microbes) and c) biotic
factors (parasitism, predation by microflora and microfauna).
2) Seed borne fungi:
a) Externally seed borne: Dormant spores on seed coat Ex: Covered smut of barley,
grain smut of jowar, bunt of wheat, etc.
b) Internally seed borne: Dormant mycelium under the seed coat or in the embryo
Ex: Loose smut of wheat (Ustilago nuda tritici)
c) Factors affecting the survival of the pathogen on/in the seed are temperature and
moisture.
3) Dormant fungal structures on dormant or active host Ex: In downy mildew of
grapevine, powdery mildew of grapevine, apple etc., The fungus mycelium may be
present in dormant state in the affected twigs or its oospores or perithecia may be
embedded in the tissues of the affected organs.
Parasitic phanerogams survive in the form of seeds, and in plant parasitic nematodes
eggs, cysts and larvae serve as over seasoning structures.
4) Survival in association with insects, nematodes and fungi
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Several important plant pathogens may survive within the insect body and over winter
therein. The corn flea beetle, Cheatocnema pulicaria carries inside its body, the corn
wilt pathogen, Xanthomonas stewartii and thus helps in over wintering.
Plant viruses like wheat mosaic, tobacco necrosis, tobacco rattle and tobacco ringspot
viruses survive with nematodes or fungi found in the soil between crop seasons.
Tobacco ringspot is associated with the nematode Xiphinema americana. The fungi,
Polymyxa graminis (Wheat soil borne mosaic & Barley yellow mosaic) and
Spongospora subterranea (Potato mop top virus) carry the viruses internally and
transmit them through the resting spore.
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LECTURE 4
DISPERSAL OF PLANT PATHOGENS
The second link in infection chain is the dissemination of plant pathogens. Transport of
spores or infectious bodies, acting as inoculum, from one host to another host at various
distances resulting in the spread of the disease, is called dispersal, dissemination or
transmission of plant pathogens. The dispersal of the pathogen or disease is important
not only for spread of plant diseases but also for continuity of the life cycle and evolution
of the pathogen. The knowledge of these methods of dispersal is essential for effective
control of plant diseases because possibilities of preventing dispersal and thereby
breaking the infection chain exist.
In fungi, productions of asexual and sexual spores follow the active vegetative growth of
the fungus in or on the host tissues and are dispersed mechanically in time and space by
various means. In bacterial diseases, the bacterial cells come out on the host surface as
ooze or the tissues may be disintegrated so that the bacterial mass is exposed and then
dispersed by various physical and biological agencies. Viral diseases which have no such
organs are transmitted by insects, mites, phanerogamic parasites, nematodes and human
beings.
The two links in the infection chain of an animate pathogen, Viz., survival through
dormant structures and the dispersal of the pathogen are very closely bound with each
other. Actually the dormant structures provide means of dispersal in time, i.e., the
pathogen is retained viable over a period of time enabling it to be transported through
physical agencies without being harmed.
The dispersal of infectious plant pathogens in space occurs through two ways:
1. Autonomous or direct or active dispersal.
2. Indirect or passive dispersal.
I) Autonomous or direct or active dispersal:
In this method the dispersal of plant pathogens takes place through soil, seed and planting
material during normal agronomic operations. There is no major role of external agencies
like insects, wind, water, etc. in this type of dispersal.
1) Seed as the source of autonomous dispersal:
Since most of the cultivated crops are raised from seed the transmission of diseases and
transport of pathogens has much importance. The dormant structures of the pathogen
(Ex: seeds of Cuscuta, Sclerotia of ergot fungus, smut sori, etc.) are found mixed with
seed lots and they are dispersed as seed contaminants. The bacterial cells or spores of
fungi present on the seed coat (such as in smuts of barley, sorghum, etc.) are transported
to long distances. Dormant mycelium of many fungi present in the seed is transmitted to
long distances. There are three types of dispersal by seed, viz., contamination of the seed,
externally seed borne and internally seed borne.
a) Contamination of the seed: Seed borne pathogens move in seed lot as separate
contaminants without being in intimate contact with the viable crop seeds. The seeds of
the pathogen or parasite and the host are mixed during harvest of the crop. In many cases,
the identity of the seeds of the two entities (host and the pathogens) is difficult to
separate. Ex: Smut of pearlmillet and ergot of rye. Smut sori and ergots mix easily with
the seed lots during harvest and threshing.
b) Externally seed borne: Close contact between structure of the pathogen and seeds
is established where the pathogen gets lodged in the form of dormant spores or bacteria
on the seed coat during growth of the crop or at the time of harvest and threshing. Ex:
Short smut of sorghum, bacterial blight of cotton, loose smut of barley etc.
In many pathogens the externally seed borne structures such as smut spores can persist
for many years due to their inherent capacity for long survival. Ex: The spores of Tilletia
caries (Stinking smut of wheat) remain viable even after 18 years and those of Ustilago
avenae (Oat smut) for 13 years.
13
c) Internally seed borne: The pathogen may penetrate into the ovary and cause infection
of the embryo while it is developing. They become internally seed borne. Ex: Loose
smut of wheat.
Differentiate Seed infection and infestation
Seed infection: The seed in infected only when the pathogen has grown in or on it for
sometime and established its relationship with the seed tissues. Ex: Loose smut of wheat,
where the fungus grows in the embryonic tissues and becomes dormant when the seed
enters dormancy.
Seed infestation: When the fungus or the pathogen is present on the seed coat and in the
seed lot, it is only transport of the pathogen and the seed is infested.
2) Soil as a means of autonomous dispersal: Soil borne facultative saprophytes or
facultative parasites may survive through soil. The dispersal may be by movement of
pathogen in the soil or by its growth in soil or by movement of the soil containing the
pathogen. The former is known as dispersal in soil while the later is called dispersal by
soil.
a) Dispersal in soil: The following are the three stages of dispersal in soil
i) Contamination of soil: Contamination of the soil takes place by gradual spread of the
pathogen from an infested area to a new area.
ii) Growth and spread of a pathogen in soil: Once the pathogen has reached the soil it can
grow and spread based on its ability to multiply and spread. Among characters of the
pathogen its adaptability to soil environment including its saprophytic survival ability
are most important. The survival ability of the pathogen is governed by high growth rate,
rapid spore germination, better enzymatic activity, capability to produce antibiotics and
tolerance to antibiotics produced by other soil-microorganisms.
On the basis of this competitive saprophytic ability the pathogens in soil can be of three
types. Specialized facultative parasites (Saprophytes) can pass their life in soil in the
absence of host plants, but they depend more on the residues of the host plant (ex:
Armillariella mellea, Ophiobolous graminis etc.). Unspecialized facultative parasites
can pass their entire life in the soil (Pythium sp., and Phytophthora sp.). The soil borne
obligate parasites such as Plasmodiophora brassicae, Synchytrium endobioticum require
the presence of active host.
iii) Persistence of the pathogen in soil: The pathogens persist in the soil as dormant
structures like oospores (Pythium, Phytophthora, Sclerospora etc.), Chlamydospores
(Fusarium), smut spores (Ustilago) and sclerotia (Rhizoctonia, Sclerotium).
b) Dispersal by the soil: The pathogen is dispersed by the soil during cultural operations
through the agricultural implements, irrigation water, workers feet etc. Propagules of
fungi and the plant debris containing the fungal and bacterial pathogens thus spread
through out the field. The transfer of soil from one place to another along with
propagating materials is the most important method of dispersal of pathogen. For
example transfer of papaya seedlings from a nursery infested with Pythium
aphanidermatum (causal agent of stem or foot rot of papaya) can introduce the pathogen
in new pits for transplanting the seedlings. Similarly grafts of fruit trees transported with
soil around their roots can transmit pathogens present in the nursery to the orchards.
3) The plant and the plant organs as a means of autonomous dispersal:
The plants, plant parts other than seed that are used for vegetative propagation, raw field
produce and plant debris that accumulates during the course of cropping constitute the
third method of autonomous dispersal. Ex: Late blight of potato was introduced in North
America and in Europe through seed tubers brought from the native source of the in
South America. Citrus canker was introduced into California from Asia. The climatic
conditions favoured its epidemic in California.
14
II) Passive or Indirect dispersal:
Passive dispersal of plant pathogens happens through animate and inanimate agents.
1) Animate agents:
a) Insects: Insects carry plant pathogens either externally (epizoic) or internally
(endozoic). They can disseminate bacteria, fungi, viruses, mycoplasmas, spiroplasmas,
rickettsia, etc.
Fungal diseases: The external transmission is of special interest in those fungi which
produce conidia, oidia and spermatia in honey secretions having attractive odours. Ex:
Sugary disease of sorghum. The spermatial oozings at the mouth of spermagonia in the
ascomycetes attract various type of insects, flies, pollinating bees and wasps which play a
dual role, viz., pollination and transmission of plant pathogens. Dutch elm disease
(Ceratostomella ulmi) is transmitted internally by elm bark beetles.
Bacterial diseases: The fire blight organism (Erwinia amylovora) and citrus canker
bacterium (Xanthomonas axonopodis pv. citri) are transmitted by flies (bees) and ants and
the later by leaf miner respectively. The cucumber wilt bacterium, Erwinia tracheiphila is
spread by the stripped cucumber beetles (Acalymma vittata) and the spotted cucumber
beetle (Diabrotica undecimipunctata). When the beetles are feeding on the diseased
plant, the bacterium contaminates the mouth parts and passes into the gut of the beetle
and over winters inside the beetle during the winter season. Thus the beetle helps the
bacteria in two ways, i.e., in their transmission and survival.
Viral diseases: More than 80 per cent of the viral and phytoplasmal diseases are spread
by different types of insects. The insect which acts as specific carriers in disseminating
the diseases are called insect vectors. Both Aphids (Aphididae) and leaf hoppers
(Cicadellidae or Jassidae) in the order Homoptera contain largest number and the most
important insect vectors of plant viruses. Certain species of mealy bugs and scale insects
(Coccoidae), whiteflies (Aleurodidae) and tree hoppers (Membracidae) in Homoptera
also transmit virus diseases. Insect vectors of plant viruses are few in true bugs
(Hemiptera), thrips (Thysanoptera), beetles (Coleoptera) and grasshoppers (Orthoptera).
S.No.
1.
2.
3.
4.
5.
6.
Vector
Virus
Aphid transmitted viruses
Beet mosaic, Lettuce mosaic, Potato
Myzus persicae
virus Y, Turnip mosaic, Beet
yellows
Bean common mosaic, Bean yellow
Acyrthosiphon pisum
mosaic, Soybean mosaic, Pea enation
mosaic
Citrus tristeza
Toxoptera citricidus
Leaf hopper transmitted viruses
Nephotettix impicticeps, N. nigropictus, Rice tungro virus
N. virescens
Rice dwarf virus
Nephotettix cincticeps, N. nigropictus
Beet curly top
Circulifer tenellus
Potato yellow dwarf
Agallia contricta
Maize chlorotic dwarf
Graminella nigrifrons
Tree hopper transmitted viruses
Tomato-pseudo curly top
Micrutalis malleifera
Plant hopper transmitted viruses
Maize mosaic
Perigrinus maidis
Rice hoja blanca
Sogatodes oryzicola
Whitefly transmitted viruses
Bhendi yellow vein mosaic, Bhendi
Bamesia tabaci
leaf curl, Chilli leaf curl, Cotton leaf
curl, Papaya leaf curl, Mungbean
yellow mosaic
Thrips transmitted viruses
15
7.
8.
9.
10.
Thrips tabaci, Frankliniella schultzei, Tomato spotted wilt virus
Scirtothrips dorsalis
Mealy bugs transmitted viruses
Cocoa swollen shoot
Planococcoides njalensis
Sugarcane spike (Phytoplasma)
Pseudococcus saccharifolii
Grass hoppers transmitted viruses
Potato virus X, Tobacco mosaic virus
Melanophus differentialis
(Mechanical transmission)
Lace bugs transmitted viruses
Beet leaf curl virus
Piesma quadratum
Root (wilt) disease of coconut
Stephanites typicus
(Phytoplasma)
Beetle transmitted viruses
Cowpea mosaic
Ceratoma trifurcata
Squash mosaic
Acalymma trivitata
Brome mosaic
Diabrotica longicornis
Mycoplasma diseases: Plant MLO’s are phloem inhabitants and those insects which are
feeding on phloem of plants transfer the MLO’s. Mycoplasmal diseases are mostly
transmitted by leaf hoppers. Ex: Sesamum phyllody (Orosious albicinctus) and little leaf
of brinjal (Hishimonas phycitis)
b) Mites: Mites belonging to the families Eryophyiidae (eryophyiid mite) and
Tetranychidae (spider mite) of class Arachnida transmit plant viruses. The genera
Abacarus, Aceria, Eriophyes and Brevipalpus are important.
Ex: Aceria cajani transmits Pigeonpea sterility mosaic virus
Aceria tulipae transmits wheat streak mosaic
c) Fungi: Some soil borne fungal plant pathogens carry plant viruses in or on their resting
spores and zoospores, and transmit them to susceptible hosts during the infection process.
Tobacco necrosis virus and Cucumber mosaic virus are carried outside the fungi, while
lettuce big vein virus is carried inside the zoospores. Many soil borne viruses are
transmitted by the members of Chytridiales and Plasmodiophorales.
Fungal transmitted viruses
S.No. Fungal vector
1.
Olpidium brassicae
2.
3.
Olpidium cucurbitacearum
Polymyxa graminis
4.
5.
6.
Polymyxa betae
Spongospora subterranea
Synchytrium endobioticum
Disease
Tobacco necrosis, Tobacco stunt,
Lettuce big vein
Cucumber necrosis
Barley yellow dwarf mosaic, Wheat
soil borne mosaic, Peanut clump
Beet necrotic yellow vein
Potato mop top
Potato virus X
d) Nematodes: Several nematodes act as vectors for transmission of fungi, bacteria and
viruses.
Bacterial diseases: The bacterium which causes yellow ear rot of wheat
(Corynebacterium tritici or Clavibacter tritici) is disseminated by ear cockle nematode,
Anguina tritici. If these two diseases appear together, a complex disease called tundu of
wheat occurs. Corynebacterium tritici is not capable of dispersal and infection unless it is
carried by Anguina tritici.
Fungal diseases: Similarly, root rot and wilt pathogens such as Phytophthora, Fusarium,
Rhizoctonia, Verticillium, etc., are disseminated by nematodes.
Viral diseases: Plant nematodes play a vital role in transmitting certain virus diseases.
Many soil borne viruses are known to be transmitted by the nematodes. Xiphenema,
Longidorous, Trichodorus and Paratrichodorus are the nematode genera belonging to
Dorylaimoidea which are known to transmit plant viruses. The nematode transmitted
viruses are divided into two groups on the basis of shape of their particles: nematode
16
transmitted polyhedral viruses (NEPO) and nematode transmitted tubular (NETU)
viruses.
NEPO viruses: These are nematode transmitted viruses with polyhedral particles.
These are generally transmitted by species of Xiphenema and Longidorus. Ex: Tobacco
ringspot virus, Tomato ringspot virus, Tomato black ring virus, Arabis mosaic virus
NETU viruses: These are nematode transmitted viruses with tubular particles. NETU
viruses are transmitted by Trichodorus and Paratrichodorous. Ex; Pea early browning
virus (Trichodorus sp.), Tobacco rattle virus (Trichodorus pachydermis)
Nematode transmitted viruses:
S.No. Nematode vector
Virus
1.
Paratrichodorus sp. & Pea
early
browning,
Trichodorus sp.
Tobacco rattle
2.
Xiphenema index
Grapevine fan leaf
3.
Tobacco ringspot, Tomato
Xiphenema americanum
ringspot
4.
Raspberry ringspot
Longidorous elongatus
Virus group
NETU group
NEPO virus
NEPO virus
NEPO virus
e) Human beings: Human beings role in dissemination of plant pathogens is more direct
than indirect. The ways and means in which human beings help in dispersal are as
follows.
¾
Transportation of seeds (seed trade): The import and export of contaminated
seeds without proper precautions lead to movement of pathogens from one country
to another or from one continent to another. The diseases which are amenable to
such transmission are mainly those that are carried in or on the propagative parts
and seed. Ex: Late blight of potato, Downy mildew of grapevine, Citrus canker,
Fusarium wilt of banana, etc.
¾
Planting diseased seed materials: Planting diseased bulbs, bulbils, corms, tubers,
rhizomes, cuttings, etc., of vegetatively propagated plants such as potato, sweet
potato, cassava, sugarcane, banana, many ornamentals and fruit trees etc., help in
dispersal of pathogens from field to field, orchard to orchard, locality to locality or
from one country to another.
¾
During adoption of normal farming practices: Human beings engaged in
preparatory cultivation, planting, irrigation, weeding, pruning etc., help in dispersal
of plant pathogens. Spores and other external structures of fungi can be carried by
workers clothing’s, shoes, and hands etc., from plant to plant and from field to field.
¾
By use of contaminated implements: Pathogens are transferred from one area to
another through implements used in various cultural operations (weeding, thinning,
hoeing etc.) in the field. Ex: Soil borne diseases such as root rot, wilt etc. Cutting
knives and pruning knives also help in dispersal from one plant to another. Ex:
Bunchy top of banana.
¾
By use of diseased grafting and budding material: Grafting and budding between
healthy and diseased plants is the most effective method of distribution of
pathogens of horticultural crops.
f)
Dispersal by phanerogamic parasites: Phanerogamic parasites transmit the viruses
by acting as a bridge between the diseased and healthy plants. Ex: Dodder
(Cuscuta California, C. campesris, C. subinclusa etc.)
Cuscuta subinclusa – Cucumber mosaic virus
Cuscuta california –
Tobacco mosaic virus
Tobacco rattle virus
Tomato spotted wilt virus
Cuscuta campestris - Tomato bushy stunt virus
17
g)
Dispersal by birds: This mode of dispersal is important in dissemination of seeds
of flowering parasites and certain fungi. In tropics, crows feeding on the fleshy,
sticky and gelatinous berries of gaint mistletoe (Dendrophthoe sp.) deposit the seeds
on the other trees with excreta. Seeds of Loranthus are disseminated by birds by
sticking on their beaks and also through excreta. Stem segments of dodder are
carried by birds for preparing their nests and thus get transported to new areas.
Moreover, spores of chestnut blight fungus, Endothea parasitica are disseminated
by more than 18 species of birds. Cleistothecia of many powdery mildew fungi are
carried by feathers of birds.
h)
Farm and wild animals: Farm animals (cattle) while feeding on diseased fodder
ingest the viable fungal propagules (spores or oospores or sclerotia) and pass out as
such in the dung. This dung when used as manure spread in the field and act as
source of inoculum. Further, soil inhabiting fungi especially sclerotia adhere to the
hoofs and legs of animals and get transported to other places.
2) Inanimate agents:
a) Wind: The dispersal of pathogens by wind is known as anemochory. Wind
transmission involves the upward air currents, velocity and the downward movements of
the wind. Wind acts as a potent carrier of propagules of fungi, bacteria and viruses.
Fungi: Usually the fungal pathogens are light in weight and are well adapted to wind
dispersal. The adaptations for wind dispersal in fungal pathogens include production of
numerous spores and conidia, discharge of spores with sufficient force, production of
very small and light spores so that they can move to long distances. Ex: Powdery mildew,
downy mildew, rusts, smuts etc.
Both short and long distance dissemination is possible by means of wind.
i) Spores adopted for short distance dissemination- sporangia of downy mildew fungi,
conidia of powdery mildew fungi and basidiospores of rust fungi. In the plains of
northern India the annual recurrence of cereal rusts is solely due to uredospores brought
by wind from the source of survival in the hills in the far north (Himalayas) and south
(Nilgiris).
ii) Spores adapted to long distance dispersal – uredospores of rust fungi,
Chlamydospores of smut fungi and conidia of Alternaria, Helminthosporium and
Pyricularia
Uredial stages of the rust fungi travel long distances through air currents and thus are
responsible for destructive epidemics over wide areas. Ex: The uredospores of Puccinia
graminis var. tritici have been detected as high as 14000 feet above infected wheat fields
(Stackman and Christensen). Similarly, Alternaria spores at 8000 feet, Puccinia
recondita and Cronartium ribicola spores at 12500 feet were reported.
Dispersal distance: In USA, uredospores of this fungus are blown from the far south
(Mexico) into Dakota and Minnesota (far north) travelling more than 1000 miles in about
two days without losing their viability. If the uredospores reach an altitude of 5000 feet,
their distance dispersal in a 30 mile per hour wind could be about 1100 miles, without
loosing viability.
Nematodes: In addition to fungi, it also helps in the dissemination of the cysts of
nematodes and also the seeds of phanerogamic parasites. Ex: Cysts of the nematode
Heterodera major, which causes molya disease of wheat and barley, are carried by dust
storms from Rajasthan to Haryana
Bacteria: Some pathogenic bacteria are carried along with the infected material to short
distances by wind. Ex: Erwinia amylovora, the causal agent of fire blight of apple and
pear, produces fine strands of dried bacterial exudates which may be broken off and are
transmitted by wind.
18
Viruses and phytoplasmas are not directly transmitted by wind, but the insect and mite
vectors that carry the viruses move to different directions and distances based on the
direction and speed of the air.
b) Water: Transmission of plant pathogens by water is called as hydrochory. Water is
less important than air in long distance transport of pathogens, but it is more efficient as
the pathogens land on the wet surface and can germinate immediately. Water
dissemination occurs mainly through surface running water and rain splash.
The surface flow of water after heavy rains or during irrigation from canals and wells
carries the pathogens to short distances. Ex: The mycelial fragments, spores or sclerotia
of fungi, Colletotrichum falcatum (red rot of sugaecane), Fusarium, Ganoderma,
Macrophomina, Pythium, Phytophthora, Sclerotium, etc., are transmitted through rain or
irrigation water. Long distance dispersal is also possible by water only when the floods
cover larger areas or when the water flows from the sources of survival of pathogens to
longer distances.
Dissemination by rain splash is also called as splash dispersal. It is one of the efficient
methods of dispersal of bacterial plant pathogens. Rain drops falling with force on sori,
pustules, cankers or even soil surface may splash the propagules in small droplets and
enable them to land on neighbouring healthy susceptible surfaces or the water droplets
may be carried to long distances by air. Ex: Bacterial leaf spot of rice (Xanthomonas
campestris pv. oryzae), Bacterial leaf streak of rice (Xanthomonas campestris pv.
oryzicola), Green ear of bajra (Sclerospora graminicola).
Fungal spores and bacteria present in the air or plant surface are washed downward by
rain splash or drops from overhead irrigation and are deposited on susceptible healthy
plants. Water not only plays an important role in the dissemination of plant pathogens,
but also helps in the growth and spore discharge of many fungi. It also helps in the spore
germination and infection process.
19
LECTURE 5 & 6
PHENOMENON OF INFECTION/ INFECTION PROCESS
It is the third link in the infection chain after survival and dispersal of inoculum.
Infection process means establishment of pathogen in the host plant. Entry and
colonization of pathogen in the host tissues is known as establishment and the infective
propagules coming in contact with the host are known as inoculum.
Inoculum potential: It is the inoculum needed for successful infection. It is a function of
inoculum density and their capacity.
Def: It is defined as the resultant of the action of environment, the vigour of pathogen to
establish an infection, susceptibility of the host and amount of inoculum present (Dimond
and Horsfall, 1960)
Or
It is defined as the energy of growth of a parasite available for infection of a host at the
surface of the host organ to be infected (Garret, 1960)
In case of specialized pathogens as rusts and powdery mildews, very few or even one
spore is capable of causing infection successfully. In case of non-specialized pathogens
such as Pythium, Phytophthora, Rhizoctonia and Sclerotium require high density of
inoculum on the surface of susceptible host for successful infection.
The success of process of infection depends on
1. Host factors
¾ Susceptibility of host: It is genetically controlled by DNA and it is an inheritable
character which is transmitted from parents to off springs.
¾ Disease proneness of the host: It is decided by the external factors such as host
nutrition, i.e., more nitrogen application makes the host more susceptible and more
potash application leads to less susceptibility.
2. Pathogen factors
¾ Virulence / aggressiveness of the pathogen: It is determined by genetic material which
is inheritable.
¾ High multiplication rate of the pathogen: Chances of infection increases with high rate
of multiplication. High birth rate and low death rate is highly essential for successful
infection.
¾ Proper inoculum potential: In case of specialized pathogens very few or even one
spore is capable of causing infection successfully, whereas, non-specialized pathogens
require high density of inoculum on the surface of susceptible host for successful
infection.
3. Environmental factors: Environmental conditions such as temperature, relative
humidity, moisture, etc., are very important for survival, dissemination and infection
process.
Process of infection can be grouped into three stages, i.e., pre-penetration, penetration
and post-penetration.
20
Stages in the development of infection or disease cycle
1. PRE-PENETRATION: Depending upon the plant pathogen activity, the plant
pathogens are classified in to 2 categories
1. Active invaders and 2.Passive invaders
Active Invaders
Passive Invaders
1. Pathogens which make an aggressive 1.No aggressive effort
effort to gain entry into intact host cells.
2. They do not require help of any external 2. Require help of external agencies like
agency to gain entry into host cells.
insect vectors or wounds caused by
agricultural implements.
3. Eg. Phyto-pathogenic fungi
3. Eg. Plant viruses
Phanerogamic parasites
Phyto-pathogenic bacteria
Plant viruses are particulate in nature and they do not have any capacity to enter the host
cell so they do not make any aggressive effort for entry, but depend on different insect
vectors for their entry into host cell. Bacteria have no dormant structures; hence no prepenetration activity except for multiplication in infection drops on the natural openings.
However, nematodes show some orientation towards root surface before actual
penetration.
In fungal pathogens, pre-penetration includes spore germination and growth of the
resulting germ tube on the surface of the host plant. Germination is essentially the change
from low metabolic rate to a high metabolic rate and involves a change from near
dormancy to intense activity; for this an energy source is needed such as a carbohydrate
or fat reserve in the propagule. Fungal invasion is chiefly by germ tubes or structures
derived from them. In some fungi like Rhizoctonia solani and Armillariella mellea, the
hypha act in a concerted way to achieve the penetration. In Rhizoctonia solani, the
fungus on coming in contact with root surface, first forms infection cushions and
appressoria and from these multiple infections takes place by means of infection pegs. In
Armillariella mellea, the fungus hyphae form the rhizomorphs (aggregation of hyphae
into rope like strands) and only these can cause infection.
Rhizomorphs
Appressorium
2. PENETRATION: Pathogens penetrate plant surfaces by direct penetration or
indirectly through wounds or natural openings. Bacteria enter plants mostly through
wounds and less frequently through natural openings. Viruses, viroids, mollicutes,
fastidious bacteria enter through wounds made by vectors. Fungi, nematodes and parasitic
higher plants enter through direct penetration and less frequently through natural
openings and wounds.
21
A. Indirect Penetration
1. Wounds: Wounds caused by farm operations, hail storms, or insect punctures, etc.,
will help in the entry of different plant pathogens into the host cells. Organisms which
cause storage diseases and ripe rots will enter through the wounds caused by farm
operations.
Ex. Rhizopus, Gloeosporium, Aspergillus, Penicilium, Colletotrichum, Diplodia, etc.
Weak parasites enter through the wounds caused by hail storms and freezing
Ex. Macrophomina phaseolina
Pathogen causing brown rot of fruits (Sclerotinia fructicola) enters through the wounds
caused by insect punctures. Similarly, causal organism of Dutch elm disease
(Ceratostomella ulmi) enters through the wounds caused by elm bark beetle.
2. Natural openings
a) Stomata: There is variation in the behaviour of germ tube at the time of penetration
through the stomata. In Puccinia graminis tritici, the uredospore germinates and forms a
germ tube which on approaching stoma swells at the tip to form an appressorium in the
stomatal aperture. From the appressorium a blade like wedge grows through the stomatal
slits and swells inside to form a sub-stomatal vesicle from which the haustoria penetrating
the cells are produced.
In Peronospora destructor infecting onion leaves, the germ tube continues to grow after
the formation of first appressorium. In Pseudoperonospora cubensis, the hyphae
penetrate the stomatal aperture and swell to form a sub-stomatal vesicle from which in
turn other hyphae grow to form haustoria in the adjacent cells of the leaves.
Mycosphaerella musicola forms a small structure called stomatopodium over the pore of
the stoma after growing for few days on the surface of the leaf. A hypha then arises from
it which grows into the sub-stomatal chamber and swells to form a vesicle, which in turn
gives rise to hyphae which invade palaside tissues.
Other examples: Xanthomonas campestris pv. malvacearum (Black arm of cotton),
Xanthomonas phaseoli (Bacterial leaf spot of green gram), Phytophthora infestans (Late
blight of potato), Albugo candida (White rust of crucifers) and uredospores of Puccinia
graminis tritici (Black stem rust of wheat).
b) Lenticels: Sclerotinia fructicola (Brown rot of fruits), Streptomyces scabies (Scab of
potato), Phytophthora arecae (Mahali disease of arecanut)
c) Hydathodes: Xanthomonas campestris pv. campestris (Black rot of crucifers)
B) Direct penetration: Most fungi, nematodes and parasitic higher plants are capable of
penetrating the host surface directly. However, the plants are provided with different
mechanisms of defense which include structural features of the host, presence of
chemical coverings on the cell walls, and anti-infection biochemical nature of the
protoplasm. Hence, the pathogen should have mechanisms to overcome these barriers for
direct penetration.
a) Breakdown of physical barriers. Viruses have no physical force or enzyme system of
their own to overcome structural or chemical barriers of the host and therefore come in
contact with the host protoplasm only through wounds. Bacteria are mostly weak
parasites and cannot employ force to effect penetration. Fungi and nematodes are the only
group of plant pathogens that employ force for direct penetration of the host. Fungi
penetrate host plants directly through a fine hypha produced directly by the spore or
mycelium or through a penetration peg produced by an appressorium. These structures
exert pressure on the surface which results in stretching of the epidermis which becomes
thin. Then the infection peg punctures it and effects its entry.
b) Breakdown of chemical barriers: the host is provided with defense mechanisms
against invasion which include i) presence of cuticular layer on the epidermis, ii) lack of
suitable nutrients for the pathogen in the host cells, iii) presence of inhibitory or toxic
substances in the host cells, iv) exudation of substances toxic to pathogen or stimulatory
22
to antagonists of the pathogen. Ex: The glands in leaf hairs of begalgram contain maleic
acidwhich is antifungal and provide resistance to infection by the rust fungus (Uromyces
ciceris arietini). Similarly, protocatecheuic acid and catechol in the red scales of onion
provide resistance to onion smudge pathogen, Colletotrichum circinans. To overcome
these physical and chemical barriers, the fungi produce various enzymes, toxins organic
acids and growth regulators.
Through non-cutinized surfaces:
a) Seedlings: Grain smut of jowar (Sphacelotheca sorghi), Loose smut of jowar
(Sphacelotheca cruenta), Downy mildew of jowar and bajra (Sclerospora graminicola),
Wheat bunt disease (Tilletia caries, Tilletia foetida)
b) Root hairs: Wilt causing fungi (Fusarium sp.), Club root of cabbage (Plasmodiophora
brassicae), Root rot of cotton (Phymatotrichum omnivorum)
c) Buds: Pea rust fungi (Uromyces pisi), Witches broom of cherries (Taphrina cerasi)
d) Flowers: Loose smut of wheat (Ustilago nuda tritici), Long smut of jowar
(Tolyposporium ehrenbergi), Bunt of rice (Neovossia horrida), Ergot of rye (Claviceps
purpurea)
e) Leaves: Basidiospores of white pine blister rust fungus (Cronartium ribicola)
germinate and grow down into branches and leaves, where aecia are produced.
d) Nectaries: Fire blight of apple (Erwinia amylovora)
e) Stalk ends: Penicillium italicum, Theilaviopsis paradoxa (Post harvest disease fungi)
Through cutinized surfaces:
a) Cuticle: Leaf spot of spinach (Cercospora beticola), early blight of solanaceous plants
(Alternaria solani), Tikka disease of groundnut (Cercospora personata)
3. POST PENETRATION
Invasion and colonization: Infection is the process by which pathogens establish contact
with the susceptible cells or tissues of the host and derive nutrients from them. A parasitic
relationship is formed between host cytoplasm and parasite cytoplasm. During infection,
pathogens grow and multiply within the plant tissues. Invasion of plant tissues by the
pathogen, and growth and reproduction of the pathogen (colonization) are two
concurrent stages of disease development.
Fungi spread into all parts of host organs, either by growing directly through the cells as
an intracellular mycelium or by growing between the cells as an intercellular mycelium.
During establishment, pathogen produces different substances which include enzymes,
toxins, growth hormones and polysaccharides which will help in colonization of the host.
In ectoparasites the main body of the pathogen lies on the surface of the host with only
feeding organs (haustoria) penetrating the tissues Ex: Most of the powdery mildew fungi.
Some fungal parasites develop both external and internal mycelium Ex: Rhizoctonia
solani. The endophytic parasites or endoparasites grow subcuticularly (Diplocarpon
rosae, black spot of rose), in parenchyma tissues (most fungal and bacterial pathogens as
well as many nematodes) or in vascular tissues (vascular wilt parasites). Some pathogens
are endobiotic, i.e., mycelium is not produced and the thallus is entirely present within a
host cell Ex: Synchytrium endobioticum.
Bacteria invade tissues intercellularly, but also grow intracellularly when parts of the cell
walls dissolve. Viruses, viroids, mollicutes and fastidious bacteria invade tissues by
moving from cell to cell intracellularly.
23
Infection caused by microbes may be local (involve single cells or few cells or small
area) or systemic (pathogen spreads and invades most or all susceptible cells and tissues
throughout the plant Ex: Sclerospora graminicola). The time interval between inoculation
and appearance of disease symptoms is called the incubation period.
Exit of the pathogen
After invasion and colonization of the host, the pathogens come out of the host to
maintain the continuity of the infection chain or disease cycle and escape death due to
overcrowding. Once the pathogens exit from the host, they survive and are disseminated
to other hosts and continue the infection cycle.
Viruses can exist only with the living protoplasm and hence disseminated through their
animate vectors like insects, fungi, nematodes, etc. The bacteria ooze out in the form of
slime on the host surface from where they can be disseminated through water and insects.
However, the fungi have the most elaborate system of exit. Most plant pathogenic fungi
grow out on the host surface and produce repeating spores (secondary inoculum), usually
asexually, under favourable conditions. The spores thus formed are disseminated through
wind, water, soil, seed, vegetative propagating material, agricultural implements, etc.
24
LECTURE 7
ROLE OF ENZYMES IN PATHOGENESIS
Enzymes are large protein molecules which catalyze all inter-related reactions in the
living cell. Most pathogens derive energy principally from enzymatic break down of food
materials from host tissue.
Composition of the cell wall: Functionally cell wall is divided into 3 regions, viz.,
middle lamella (made of pectins), primary wall (cellulose, pectic substances) and
secondary cell wall (entirely cellulose).
Middle lamella acts as intercellular cement which binds the cells together in tissue
system. Pectin or pectic substances are major chemical constituents of wall layers and
entire middle lamella, where as in other layers, cellulose is found in good amounts.
Besides these two major components, other components such as hemicelluloses, lignin
and some amount of protein is also present. Main components of cell wall are pectic
substances, cellulose, hemicelluloses, lignin and small quantity of protein.
The epidermis of plants is covered by cuticle, whose major chemical substance is cutin in
addition to cuticular wax.
Cuticular wax: Plant waxes are found as granular or rod like projections or as a
continuous layer outside / within the cuticle. Wax formation is a continuous process and
it is not a terminal phase in the development of leaf. Cuticular waxes are made up of long
chain molecules of paraffin, hydrocarbons, alcohols, ketones and acids. Most of the fungi
and parasitic higher plants penetrate wax layers by means of mechanical force alone.
Cutin: It is an insoluble polyester of unbranched derivatives of C16 and C18 hydroxy
fatty acids. Cutin is admixed with waxes on upper side and with pectin and cellulose on
the lower side. Cutinases break cutin molecules and release monomers as well as
oligomers from insoluble cutin polymer. Cutinases reaches its highest concentration at
penetrating point of the germ tube and at infection peg of appressorium forming fungi
Ex: Colletotrichum gloeosporioides, Sphaerotheca pannosa, Venturia inaequalis,
Helminthosporium victoriae.
Pectic substances: These are major components of middle lamella (intercellular cement
that holds in place the cells of plant tissues). They also make up a large portion of
primary cell wall in which they form an amorphous gel filling the spaces between
cellulose microfibrils. Pectic substances are polysaccharides consisting mostly of dgalactouronic acid units with α-1,4-glycosidic bonds. These chains are esterified with
methyl groups or linked with other carboxyl groups in calcium and magnesium salt
bridges.
Pectic substances are of three types, namely, pectic acid (non methylated units), pectinic
acid (<75% methylated galacturonan units) and pectin (>75% methylated units). Term
protopectin is used to denote substances which are soluble in water and upon restricted
hydrolysis yields pectinic acid.
25
The enzymes that degrade pectic substances are known as pectinases or pectolytic
enzymes. Pectinases and pectolytic enzymes are pectin methyl esterases (PME’s),
polygalactouronases (PG’s) and pectin lyases (PL’s).
1. Pectin methyl esterases: Breaks ester bonds and removes methyl groups from pectin
leading to the formation of pectic acid and methanol (CH3OH).
2. Polygalacturonases: Split pectin chain by adding a molecule of water and breaks the
linkage between two galacturonan units. These enzymes catalyze reactions that break α1,4-glycosidic bonds.
3. Pectin lyases: Split pectin chain by removing a molecule of water from the linkage,
thereby breaking it and releasing products with unsaturated double bonds.
These pectin enzymes can be exopectinases (break only terminal linkage) or
endopectinases (break pectin chain to random sites). Pectin degradation results in
liquefaction of the pectic substances and weakening of cell walls, leading to tissue
maceration
Ex: Soft rot bacterium, Erwinia caratovora subsp. caratovora and other fungi like
Botrytis cinerea, Sclerotium rolfsii, etc.
Cellulose: Cellulose is a polysaccharide, made of chains of β-D-glucopyranose units
(where C1 is linked to C4). Glucose chains are held by hydrogen bonds. Cellulose occurs
in all higher plants as the skeletal substance of cell walls in the form of microfibrils.
Primary and secondary wall consists of a matrix in which a large number of microfibrils
are embedded. These microfibrils are like bundles of iron bars in a reinforced concrete
building. In some parts of microfibrils the chains are arranged in an orderly fashion
attaining crystalline form, when arranged in less orderly fashion, it attains amorphous
form. If the proportion of crystalline portion is more, the resistance of the host to
pathogen is more. The space between microfibrils and between micelles or cellulose
chains is filled with pectins, hemicelluloses and also lignin at maturity.
Cellulose is insoluble in crystalline form (native form), and soluble in amorphous form
(modified cellulose). The enzymatic breakdown of cellulose results in final production of
glucose molecules.
Cellulose is degraded by cellulases. Cellulase one (C1) attacks native cellulose by
cleaving cross-linkages between chains. A second cellulase (C2) also attacks native
cellulose and breaks into shorter chains. These shorter chains are then attacked by Cx
enzyme, which degrade them into disaccharide, cellobiose. Finally cellobiose is degraded
by the enzyme, β-glucosidase into glucose.
26
Cellulase degrading enzymes play a role in softening and degradation of cell wall
material and facilitate easy penetration and spread of pathogen in the host.
Ex: Basidiomycetes fungi
Hemicellulose: These are the major constituents of primary cell wall and also seen in
middle lamella and secondary cell wall. The hemicellulose polymers include primarily
xyloglucan but also glucomannans, galactomannans, arabinogalactans, etc.
Hemicelluloses link the ends of pectic polysaccharides and various points of the cellulose
microfibrils.
Hemicellulases degrade hemicelluloses and depending on the monomer released from
polymer on which they act, they are termed as xylanase, galactanase, glucanase,
arabinase, mannose, and so on. Ex: Sclerotinia sclerotiorum, Sclerotinia fructigena.
Lignin: Lignin is found in the middle lamella, as well as in the secondary cell wall of
xylem vessels and the fibres that strengthen plants. It is an amorphous, three-dimensional
polymer made up of basic structural unit, phenylpropanoid. Lignin forms by oxidative
condensation (C-C and C-O bond formation) between phenylpropanoid units or
substituted cinnamyl alcohols (p-coumaryl alcohol, coniferyl alcohol and sinapyl
alcohol). White rot fungi (Basidiomecetes) secrete one or more ligninases which enable
them to utilize lignin. Ex: Xylaria, Chaetomium, Alternaria, Cephalosporium, etc.
Cell wall proteins: Cell wall proteins are similar to other proteins, except that they are
rich in aminoacid, hydroxy proline. Five classes of structural proteins are found in cell
walls: extensins, proline-rich proteins (PRP’s), glycine-rich proteins (GRP’s),
Solanaceous lectins and arabinogalactan proteins (AGP’s). Proteins are degraded by
means of enzymes, proteases or proteinases or peptidases.
Lipids: Various types of lipids occur in all plant cells. The most important ones are
phospholipids and glycolipids. These lipids contain fatty acids, which may be saturated
or unsaturated. Lipolytic enzymes, called lipases (phospholipases, glycolipases)
hydrolyze lipids and release fatty acids.
Starch: Starch is the main reserve polysaccharide found in plant cells. It is a glucose
polymer and exists in two forms: amylose, a linear molecule, and amylopectin, a highly
branched molecule. Starch is degraded by enzyme, amylases.
27
LECTURE 8
ROLE OF TOXINS IN PLANT PATHOGENESIS
Def: Toxin can be defined as a microbial metabolite excreted (exotoxin) or released by
lysed cells (endotoxin) which in very low concentration is directly toxic to the cells of
the suscept (host).
The term toxin is used for a product of the pathogen, its host, or pathogen host interaction
which even at very low concentration directly acts on living host protoplasm to influence
disease development or symptom expression.
Toxins are different from enzymes in that they do not attack structural integrity of host
tissues but affect the metabolism of the host because the toxins will act on protoplast of
the cell.
Toxin hypothesis (Luke and Wheeler, 1955):
1. A toxin should produce all symptoms characteristic of the disease
2. Sensitivity to toxin will be correlated with susceptibility to pathogen
3. Toxin production by the pathogen will be directly related to its ability to cause disease.
Except, victorin, the toxic metabolite of Cochliobolus victoriae, the vast majority of
toxins associated with plant diseases fail to exhibit all the above characters.
Classification of toxins (Wheeler and Luke, 1963)
According to the source of origin, toxins are divided into 3 broad classes namely,
pathotoxins, vivotoxins and phytotoxins.
1. Pathotoxins: These are the toxins which play a major role in disease production and
produce all or most of the symptoms characteristic of the disease in susceptible plants.
Most of these toxins are produced by pathogens during pathogenesis.
Ex: Victorin: Cochliobolus victoriae (Helminthosporium victoriae), the causal agent of
Victoria blight of oats. This is a host specific toxin.
Other examples:
a) Selective
T- toxin: Helminthosporium maydis race T
HC-toxin: Helminthosporium carbonum
HS- toxin: Helminthosporium sacchari
Phyto-alternarin: Alternaria kikuchiana
PC- toxin: Periconia circinata
b) Non-selective
Tentoxin: Alternaria tenuis
Tabtoxin or wild fire toxin: Pseudomonas tabaci
Phaseolotoxin: Pseudomonas syringae pv. phaseolicola
c) Produced by plant or plant X pathogen interaction
Amylovorin: Erwinia amylovora (Fire blight of apple and pears)
2) Phytotoxins: These are the substances produced in the host plant due to host-pathogen
interactions for which a causal role in disease is merely suspected rather than established.
These are the products of parasites which induce few or none of the symptoms caused by
the living pathogen. They are non-specific and there is no relationship between toxin
production and pathogenicity of disease causing agent.
Ex: Alternaric acid – Alternaria solani
3) Vivotoxins: These are the substances produced in the infected host by the pathogen
and / or its host which functions in the production of the disease, but is not itself the
initial inciting agent of the disease.
Fusaric acid – Wilt causing Fusarium sp.
28
Lycomarasmin – Fusarium oxysporum f.sp. lycopersici
Piricularin – Pyricularia oryzae
Classification based on specificity of toxins
1. Host specific / Host selective toxins: These are the metabolic products of the
pathogens which are selectively toxic only to the susceptible host of the pathogen
Ex: Victorin, T-toxin, Phyto-alternarin, Amylovorin
2. Non-specific / Non-selective toxins
These are the metabolic products of the pathogen, but do not have host specificity and
affect the protoplasm of many unrelated plant species that are normally not infected by
the pathogen
Ex: Ten-toxin, Tab-toxin, Fusaric acid, Piricularin, Lycomarasmin and Alternaric acid
Differentiate host – specific and non-host specific toxins
Host specific
1. Selectively toxic only to susceptible
host of the pathogen
Non-host specific
1. No host specificity and can also affect
the physiology of those plants that are
normally not infected by the pathogen
2. Primary determinants of disease
2. Secondary determinants of disease
3. Produce all the essential symptoms
of the disease
3. Produce few or none of the symptoms of
the disease
4. Ex: Victorin, T- toxin
4. Ex: Tentoxin, Tabtoxin
Effect of toxins on host tissues
A) Changes in cell permeability: Toxins kill plant cells by altering the permeability of
plasma membrane, thus permitting loss of water and electrolytes and also unrestricted
entry of substances including toxins. Cellular transport system, especially, H+ / K+
exchange at the cell membrane is affected.
B) Disruption of normal metabolic processes
¾ Increase in respiration due to disturbed salt balance
¾ Malfunctioning of enzyme system Ex: Piricularin inhibits polyphenol oxidase
¾ Uncoupling of oxidative phosphorylation
C) Interfere with the growth regulatory system of host plant Ex: Restricted development
of roots induced by Fusarium moniliforme
ROLE OF GROWTH REGULATORS IN PLANT PATHOGENESIS
Growth regulators
Growth regulators are of two types
1. Growth promoting substances and 2. Growth inhibiting substances
Auxins, gibberellins and cytokinins are growth promoting substances, whereas, dormin,
ethylene and abscissic acid are growth inhibiting substances. The imbalance in growth
promoting and growth inhibiting substances causes hypertrophy (excessive increase in
cell size) and atrophy (decrease in cell size). Symptoms may appear as tumors, galls,
knots, witches broom, stunting, excessive root branching, defoliation and suppression of
bud growth.
1. Growth promoting substances:
a) Auxins: Indole-3-acetic acid (IAA) is the naturally occurring auxin. It is continuously
produced in young meristematic tissue and moves rapidly to older tissues. If auxin
concentration is more, its concentration is reduced by the enzyme, IAA oxidase.
Functions: IAA regulates cell elongation and differentiation, also affects permeability of
the membrane, increases respiration, and promotes synthesis of mRNA.
29
How disease is induced?
Increased IAA results in hypertrophy and decreased IAA results in atrophy. Increased
IAA may be due to inhibition of IAA oxidase.
Ex: Ralstonia solanacearum (Pseudomonas solanacearum), the causal agent of wilt of
Solanaceous plants, induces a 100 fold increase in IAA level in diseased plants. Increased
plasticity of cell walls as a result of high IAA levels renders the pectin, cellulose and
protein components of the cell wall more accessible to pathogen degradation. Increase in
IAA levels may also inhibit lignifications of tissues.
Increased IAA levels have been reported in plants infected with the following pathogens
Phytophthora infestans (Late blight of potato), Ustilago maydis (Maize smut),
Plasmodiophora brassicae (Club root of crucifers), Sclerospora graminicola (Downy
mildew of sorghum), Agrobacterium tumefaciens (Crown gall of apple), and
Meloidogyne (Root knot nematode).
b) Gibberellins: First isolated from Gibberella fujikuroi (Conidial stage: Fusarium
moniliforme), the causal agent of bakanae or foolish seedling disease of rice. Infected
seedlings show abnormal elongation due to excessive elongation of internodes. Best
known gibberellin is Gibberellic acid.
Functions: Cell elongation, stem and root elongation, promote flowering and growth of
fruits. It also induces IAA synthesis. IAA and GA act synergistically. Ex: Sclerospora
sacchari, the causal agent of downy mildew of sugarcane induces GA production.
c) Cytokinins: Kinetin was the first compound isolated from herring sperm DNA and
does not occur naturally in plants. Cytokinins, such as zeatin and isopentenyl adenosine
(IPA) have been isolated from plants
Functions: Cytokinins are necessary for cell growth and differentiation. It inhibits
breakdown of proteins and aminoacids and thereby inhibit senescence and they have the
capacity to direct the flow of aminoacids and other nutrients towards high cytokinin
concentration. Cytokinin activity increases in club root, in crown galls and in rust
infected bean leaves. Ex: Green islands are formed around infection in bean (Phaseolus
vulgaris) leaves infected by Uromyces phaseoli.
2. Growth inhibiting substances
a) Ethylene (CH2=CH2): Ethylene exerts a variety of effects on plants, viz., chlorosis,
leaf abscission, epinasty, stimulation of adventitious roots, fruit ripening and increased
permeability of cell membranes.
Ex: Ethylene is involved in premature ripening of fingers in banana infected by
Pseudomonas solanacearum, the causal agent of moko disease of banana. Ethylene was
also detected in leaf epinasty symptom of the vascular wilt syndrome. Ex: Fusarium
oxysporum f.sp. lycopersici (Wilt in tomato).
b) Abscissic acid: It exerts dormancy in seeds, closure of stomata, inhibition of seed
germination and growth and stimulated germination of fungal spores. It is one of the
factors involved in stunting of plants.
c) Dormin / Abscissin II: Dormin induces dormancy by converting developing leaf
primordia of a bud into bud scales. It acts as an antagonist of gibberellins and masks the
effect of IAA. However, the exact role of dormin is not known.
ROLE OF POLYSACCHARIDES IN PATHOGENESIS
Polysaccharides: Fungi, bacteria and nematodes release varying amounts of
mucilaginous substances that coat their bodies and provide interface between the outer
surface of the micro-organism and its environment. The role of slimy polysaccharides is
of utmost importance in wilt diseases. In the vascular wilts, large polysaccharide
molecules released by the pathogen in the xylem causes mechanical blockage of vascular
bundles and initiate wilting.
Ex: Ralstonia solanacearum (Bacterial wilt of Solanaceous plants)
30
LECTURE 9, 10 & 11
DEFENSE MECHANISM IN PLANTS
In general plants defend themselves against pathogens by two ways: structural
or morphological characteristics that act as physical barriers and biochemical reactions
that take place in cells and tissues that are either toxic to the pathogen or create
conditions that inhibit the growth of the pathogen in the plant.
I. Structural defense mechanisms: These may be pre-existing, which exist in the plant
even before the pathogen comes in contact with the plant or induced, i.e, even after the
pathogen has penetrated the preformed defense structures, one or more type of structures
are formed to protect the plant from further pathogen invasion.
A) Pre-existing structural defense structures
These include the amount and quality of wax and cuticle that cover the epidermal cells
and the size, location and shapes of natural openings (stomata and lenticels) and presence
of thick walled cells in the tissues of the plant that hinder the advance of the pathogen.
i) Waxes: Waxes on leaf and fruit surfaces form a hydrophobic or water repellent surface
preventing the germination of fungi and multiplication of bacteria.
ii) Cuticle and epidermal cells: A thick cuticle and tough outer wall of epidermal cells
may increase resistance to infection in diseases in which the pathogen enters its host only
through direct penetration. Ex: Disease resistance in Barbery species infected with
Puccinia graminis tritici has been attributed to the tough outer epidermal cells with a
thick cuticle. In linseed, cuticle acts as a barrier against Melampsora lini.
The silicification and lignifications of epidermal cells offers protection against
Pyricularia oryzae and Streptomyces scabies in paddy and potato, respectively.
iii) Sclerenchyma cells: The sclerenchyma cells in stems and leaf veins effectively
blocks the spread of some fungal and bacterial pathogens that cause angular leaf spots.
iv) Structure of natural openings:
a) Stomata: Most of the pathogens enter plants through natural openings. Some
pathogens like stem rust of wheat can enter its host only when the stomata are open. The
wheat varieties (Cultivar, Hope) in which stomata open late in the day are resistant as the
germ tubes of the spores germinating in the night dew desiccate owing to evaporation of
the dew before stomata begin to open. This can also be called as functional resistance.
The structure of stomata provides resistance to penetration by certain plant pathogenic
bacteria.
Ex: The citrus variety, szinkum, is resistant to citrus canker because it posses a broad
cuticular ridge projecting over the stomata and a narrow slit leading to the stomatal cavity
thus preventing the entry of bacterial and fungal spores into the interior of the leaf.
b) Lenticels: The shape and internal structure of lenticels can increase or decrease the
incidence of fruit diseases. Small and suberised lenticels will offer resistance to potato
scab pathogen, Streptomyces scabies.
B) Post-infectional structural defense mechanisms/Induced structural barriers:
These may be regarded as histological defense barriers (cork layer, abscission layers and
tyloses) and cellular defense structures (hyphal sheathing).
i) Histological defense structures
a) Cork layer: Infection by fungi, bacteria, some viruses and nematodes induce plants to
form several layers of cork cells beyond the point of infection and inhibits the further
invasion by the pathogen beyond the initial lesion and also blocks the spread of toxin
substances secreted by the pathogen. Furthermore, cork layers stop the flow of nutrients
and water from the healthy to the infected area and deprive the pathogen of nourishment.
Ex: Potato tubers infected by Rhizoctonia; Prunus domestica leaves attacked by
Coccomyces pruniphorae.
31
b) Abscission layers
An abscission layer consists of a gap formed between infected and healthy cells of a
leaf surrounding the locus of infection due to the disintegration of the middle lamella of
parenchymatous tissue.
Gradually, infected area shrivels, dies, and sloughs off, carrying with it the pathogen.
Abscission layers are formed on young active leaves of stone fruits infected by fungi,
bacteria or viruses.
Ex: Xanthomonas pruni, and Closterosporium carpophylum on peach leaves
c) Tyloses
Tyloses are the overgrowths of the protoplast of adjacent living parenchymatous cells,
which protrude into xylem vessels through pits. Tyloses have cellulosic walls and are
formed quickly ahead of the pathogen and may clog the xylem vessels completely
blocking the further advance of the pathogen in resistant varieties. In susceptible
varieties, few or no tyloses are formed ahead of pathogen invasion.
Ex: Tyloses form in xylem vessels of most plants under invasion by most of the vascular
wilt pathogens.
ii) Cellular defense structures:
Hyphal sheathing: The hyphae penetrating the cell wall and growing into the cell lumen
are enveloped by a cellulosic sheath (callose) formed by extension of cell wall, which
become infused with phenolic substances and prevents further spread of the pathogen.
Ex: Hyphal sheathing is observed in flax infected with Fusarium oxysporum f.sp. lini.
32
II) Biochemical defense mechanisms: These can be classified as pre-existing and
induced biochemical defenses.
1) Pre-existing chemical defenses:
a) Inhibitors released by the plant in its environment:
Plants exude a variety of leaf and root exudates which contain aminoacids, sugars,
glycosides, organic acids, enzymes, alkaloids, flavones, toxic materials, inorganic ions
and also certain growth factors. The inhibitory substances directly affect micro-organisms
or encourage certain groups to dominate the environment which may act as antagonists to
pathogen.
¾ Ex 1: Tomato leaves secrete exudates which are inhibitory to Botrytis cinerea
¾ Ex 2: Red scales of red onion contain the phenolic compounds, protocatechuic acid
and catechol, which diffuse out to the surface and inhibits the conidial germination of
onion smudge fungus, Colletotrichum circinans. However, these fungitoxic phenolic
compounds are missing in white scaled onions.
¾ Ex 3: Resistant varieties of apple secrete waxes on the leaf surface which prevents the
germination of Podosphaera leucotricha (powdery mildew of apples).
¾ Ex 4: In Cicer arietinum (chickpea), the Ascochyta blight resistant varieties have more
glandular hairs which have maleic acid which inhibit spore germination.
¾ Ex 5: Resistant varieties of linseed secrete HCN in roots which are inhibitory to
linseed wilt pathogen, Fusarium oxysporum f.sp. lini.
¾ Ex 6: Root exudates of marigold contain α-terthinyl which is inhibitory to nematodes.
¾ Ex 7: Chlorogenic acid present in sweet potato, potato and carrot inhibits
Ceratocystis fimbriata. Similarly caffeic acid and phloretin are present in sweet
potato and apple, respectively.
b) Inhibitors present in plant cells before infection:
¾ Antimicrobial substances pre-existing in plant cells include unsaturated lactones,
cyanogenic glycosides, Sulphur containing compounds, phenols, phenolic glycosides
and saponins
¾ Several phenolic compounds, tannins, and some fatty acid like compounds such as
dienes, which are present in high concentrations in cells of young fruits, leaves or
seeds are responsible for the resistance of young tissues to Botrytis. These compounds
are potent inhibitors of many hydrolytic enzymes.
Ex: Chlorogenic acid in potato inhibits common scab bacteria, Streptomyces scabies,
and to wilt pathogen, Verticillium alboatrum
¾ Saponins have antifungal membranolytic activity which excludes fungal pathogens
that lack saponinases. Ex: Tomatine in tomato and Avenacin in oats
¾ Similarly, lectins, which are proteins that bind specifically to certain sugars and occur
in large concentrations in many types of seeds, cause lysis and growth inhibition of
many fungi.
¾ Plant surface cells also contain variable amounts of hydrolytic enzymes such as
glucanases and chitinases which may cause breakdown of pathogen cell wall.
2) Post inflectional or induced defense mechanisms:
a) Phytoalexins (Phyton = plant; alexin = to ward off)
¾ Muller and Borger (1940) first used the term phytoalexins for fungistatic compounds
produced by plants in response to injury (mechanical or chemical) or infection.
¾ Phytoalexins are toxic antimicrobial substances produced in appreciable amounts in
plants only after stimulation by phytopathogenic micro-organisms or by chemical or
mechanical injury.
33
¾ Phytoalexins are not produced by uninfected healthy plants, but produced by healthy
cells adjacent to localized damaged or necrotic cells in response to materials diffusing
from the infected cells. These are not produced during compatible biotrophic
infections.
¾ Phytoalexins accumulate around both resistant and susceptible necrotic tissues.
However, resistance occurs when one or more phytoalexins reach a concentration
sufficient to restrict pathogen development.
Characteristics of phytoalexins
1. Fungitoxic and bacteriostatic at low concentrations.
2. Produced in host plants in response to stimulus (elicitors) and metabolic products.
3. Absent in healthy plants
4. Remain close to the site of infection.
5. Produced in quantities proportionate to the size of inoculum.
6. Produced in response to the weak or non-pathogens than pathogens
7. Produced within 12-14 hours reaching peak around 24 hours after inoculation.
8. Host specific rather than pathogen specific.
Synthesis and accumulation of phytoalexins are shown in diversified families, viz.,
Leguminosae, Solanaceae, Malvaceae, Chenopodiaceae, Convolvulaceae, Compositae
and Graminaceae.
S.No.
1
2
3
4
5
6
7
Phtoalexin
Pisatin
Phaseolin
Rishitin
Gossypol
Cicerin
Ipomeamarone
Capsidol
Host
Pea
French bean
Potato
Cotton
Bengalgram
Sweet potato
Pepper
Pathogen
Monilinia fructicola
Sclerotinia fructigena
Phytophthora infestans
Verticillium alboatrum
Ascochyta rabiei
Ceratocystis fimbriata
Colletotrichum capsici
b) Hypersensitive response (HR)
¾ The term hypersensitivity was first used by Stakman (1915) in wheat infected by rust
fungus, Puccinia graminis.
¾ The hypersensitive response is a localized induced cell death in the host plant at the
site of infection by a pathogen, thus limiting the growth of pathogen. In the infected
plant part, HR is seen as water soaked large sectors which subsequently become
necrotic and collapsed.
¾ HR occurs only in incompatible host-pathogen combinations. HR may occur
whenever virulent strains or races of pathogens are injected into non-host plants or
into resistant varieties, and when avirulent strains or races of pathogens are injected
into susceptible cultivars.
¾ HR is initiated by the recognition of specific pathogen-produced signal molecules,
known as elicitors. Recognition of the elicitors by the host results in altered cell
functions leading to the production of defense related compounds.
The most common new cell functions and compounds include:
¾ A rapid burst of oxidative reactions
¾ Increased ion movement, especially of K+ and H+ through cell membrane
¾ Disruption of membranes and loss of cell compartmentalization
¾ Cross-linking of phenolics with cell wall components and strengthening of plant cell
wall
¾ Production of antimicrobial substances such as phytoalexins and pathogenesis-related
proteins (such as chitinases)
Cellular responses during HR
¾ In many host-pathogen combinations, as soon as the pathogen establishes contact with
the cell, the nucleus moves toward the invading pathogen and soon disintegrates.
¾ Brown resin like granules form in the cytoplasm, first around the point of penetration
of pathogen and then throughout the cytoplasm
¾ As the browning discolouration of the cytoplasm continues and death sets in, the
invading hypha begins to degenerate and further invasion is stopped.
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c) Plantibodies: Transgenic plants have been produced which are genetically engineered
to incorporate into their genome, and to express foreign genes, such as mouse genes that
produce antibodies against certain plant pathogens. Such antibodies, encoded by animal
genes, but produced in and by the plant, are called plantibodies. Ex: Transgenic plants
producing plantibodies against coat protein of viruses, such as, artichoke mottle crinkle
virus have been produced.
35
LECTURE 12
PLANT DISEASE EPIDEMIOLOGY
Epiphytology or Epidemiology of plant diseases is essentially a study of the rate of
multiplication of a pathogen and spread of the disease caused by it in a plant population.
Epidemiology deals with outbreaks and spread of diseases in a population.
Importance of epidemiology:
Knowledge of epidemiology is useful in forecasting of a disease and also for the
management of a disease
Terms compound interest and simple interest diseases were given by Vanderplank
(1963) in his book “Plant Disease Epidemics and control”
S.No. Compound interest disease/ polycyclic
1
Rate of increase of disease is
mathematically analogous to compound
interest in money (Interest is added
periodically to the capital; interest gets
interest)
2
Pathogen produces spores at rapid rate
3
4
5
6
Simple interest disease/Monocyclic
Rate of increase of disease is
mathematically analogous to simple
interest in money (Interest is added
only at the end; interest does not get
interest)
Pathogen produces spores at very
slow rate
Propagules disseminate by air
Propagules disseminate by soil or
seed
Incubation period and sporulation period Incubation period and sporulation
is short
period is long
There are several generations of the There is only one generation of the
pathogen in the life of a crop
pathogen in the life of a crop
Ex: Rusts of cereals
Ex: Smuts of wheat, barley &
sorghum
Disease Triangle: The interactions of three components of disease, i.e., the host,
pathogen and environment, can be visualized as a disease triangle. The length of each
side is proportional to the sum total of the characteristics of each component that favour
disease.
The interaction of susceptible host plant, virulent pathogen and favourable environmental
conditions leads to the development of the disease.
Disease Pyramid: The disease triangle can be expanded to include two more
components, time and humans. The amount of each of the three components of disease
and their interaction in the development of the disease are affected by fourth component,
time. Thus addition of time component to the disease triangle results into a tetrahedron
or disease pyramid. The effect of time on disease development becomes apparent when
we consider the importance of time of year, the duration and frequency of favourable
temperature and rain, the time of appearance of the vector, the duration of the cycle of a
particular disease. If the four components of disease pyramid could be quantified, its
volume would be proportional to the amount of disease on a plant or in plant population.
Humans affect disease development in various ways. They affect the type of plants grown
in an area, their level of resistance, time of planting, density of planting, etc.
36
Essential components/conditions for an Epiphytotic:
1. Host factors
2. Pathogen factors
3. Environmental factors
1. Host factors
i) Distance of susceptible plants from the source of primary inoculum: Longer the
distance from the source of survival of the pathogen, longer will be the time required for
the buildup of an Epiphytotic in a susceptible crop.
ii) Abundance and distribution of susceptible hosts: Continuous cultivation of a
susceptible variety over a large contiguous area helps in the buildup of the inoculum and
improves the chances of epiphytotics.
iii) Disease proneness in the host due to environment: Susceptibility is genetically
controlled but the disease proneness in the plant to get infected can be induced by
environment and other factors (Host nutrition, excessive application of nitrogenous
fertilizers, etc).
iv) Presence of suitable alternate or collateral hosts: These host plants help in the
survival of inoculum of different pathogens in off season. Presence of Barbery which is
an alternate host to Puccinia graminis tritici helps in the heterogenous infection chain.
Presence of grass hosts helps in the survival of Pyricularia oryzae in the off-season.
2. Pathogen factors:
i) Presence of virulent/aggressive isolate of a pathogen: For any epiphytotic, rapid cycle
of infection is essential, and successful infection can be caused only by virulent isolates
of the pathogen.
ii) High birth rate: The fungi that assume epiphytotic form invariably have the capacity
to produce enormous quantity of spores that are adapted to long distance dissemination in
a short time.
iii) Low death rate of the pathogen: Epiphytotics is attributed to low death rate of the
pathogens in those in which the causal agent is systemic and protected by the plant
tissues.
iv) Easy and rapid dispersal of the pathogen: The ability of a pathogen to cause
epiphytotics is much more dependent on its dispersal rate. The units of propagation need
to be dispersed by external agencies, if epiphytotics are to develop.
Ex: Fungal spores disseminated by wind, water, etc
Viruses disseminated by insect vectors
Bacteria dispersed through rain splashes and water
v) Adaptability of the pathogen: Most of the pathogens causing epiphytotics adapt
themselves to various adverse conditions.
3. Weather factors: Assuming that a particular fungus meets all the above requirements
for causing an epidemic, the infection, invasion and development of epidemic may not
occur if weather is not favourable for the germination of spores. Congenial environmental
conditions, viz., optimum weather conditions for sporulation, dispersal, infection and
survival of pathogen, are very important.
Weather conditions such as, optimum temperature, moisture, light, etc., are very essential
for the development of an epidemics.
Science which deals with the relationship between weather and epiphytotics is called
metereopathology.
REMOTE SENSING
Remote sensing is estimating an object/phenomenon without being in physical contact
with it. Remote sensing is a science/art that permits us to obtain information about an
object/a phenomenon through analysis of data obtained through sensory devices without
being in physical contact with that object.
37
Objectives of remote sensing in plant Pathology
1. Assessment of disease over a vast area
2. To know the relationship of diseases and environment
3. To know the origin and development of epidemics
4. Quantitative assessment of the disease
Remote sensing techniques of importance to Plant Pathology
1. Aerial photography and 2. Satellite remote sensing
1. Aerial photography: Aerial photography can detect objects on land over a larger area.
Colwell (1956) first used remote sensing technique for monitoring stem rust of wheat. He
showed that panchromatic colour and especially infrared aerial photography could be
used to detect rusts and viral diseases of small grains and certain diseases of citrus. Later,
infrared photography was used in England for late blight of potato.
The key to distinguish diseased and healthy parts of a crop is to use appropriate film or
filter combinations. The main film types used are panchromatic, infrared, normal colour
and colour infrared. The infrared films are preferred because of their superior sensitivity
to visible light and to near infrared wavelengths of radiation (700-900 mµ). The colour
infrared or Ektachrome Aero Infrared (Camouflage Detection Film) is superior as it can
show the difference between diseased and healthy patches of plants in colour. The
healthy foliage is highly reflective to the infrared wavelengths and appears red on this
film whereas blighted or diseased foliage has low infrared reflectance and does not
appear red in the photograph.
2. Satellite Imaging
Weather satellites
Often cyclones create heavy clouds with rains and an anti-cyclone creates a cloudless
sky. All these can be effectively monitored by weather satellites. Sequential pictures
show the movement of these systems before they arrive in an area. Therefore by
monitoring epidemic favouring systems using a satellite, the disease occurrence on the
field can be monitored. Ex: The spread and deposition of stem rust pathogen of wheat is
influenced by definite synoptic weather conditions called Indian stem rust rules.
Earth resources technology satellites (LANDSAT, 1972, USA)
LANDSAT covers the entire globe every 18 days scanning the same area at a fixed time.
The scanned data is compared for any major differences happened within 18 days.
Nagarajan utilized LANDSAT infrared spectral bands 6 (0.7-0.8µm) and 7 (0.8-1.1µm)
to differentiate healthy wheat crop of India and severe yellow rust affected crop of
Pakistan.
Examples: Coconut root rot and wilt, black stem rust of wheat, citrus canker
Advantages of Remote sensing
1. Reveals pattern of disease incidence, intensity and development over large area
2. Data generated by remote sensing is amenable to multidisciplinary approach
3. Gives synoptic view of large areas
4. Data generated is on a permanent scale and is unbiased
5. Data acquisition is fast compared to traditional methods and data analyzed is
effectively utilized
6. Satellite data (ERTS) obtains information of an area periodically so that the
information can be updated.
7. It frequently poses questions for ground investigators which cannot be generated by
ground parties
38
LECTURE 13
PRINCIPLES OF PLANT DISEASE MANAGEMENT
Management: It conveys a concept of continuous process which is based not only on the
principle of eradication of the pathogen but mainly on the principle of minimizing the
damage or loss below economic injury level.
Importance: Plant diseases are important because of the losses (qualitative and
quantitative) they cause. Loss may occur at any time between sowing of the crop and
consumption of the produce. Measures taken to prevent the incidence of the disease,
reduce the amount of inoculum that initiates and spreads the disease and finally minimize
the loss caused by the disease are called as management practices.
Essential considerations in plant disease Management:
1. Benefit-cost ratio
2. Procedures for disease control should fit into general schedule of operations of crop
production
3. Control measures should be adopted on a co-operative basis over large adjoining
areas. This reduces frequency of applications, cost of control and increases chances of
success of control measures
4. Knowledge aspects of disease development is essential for effective economical
control. Information is needed on the following aspects
a. Cause of a disease
b. Mode of survival and dissemination of the pathogen
c. Host parasite relationship
d. Effect of environment on pathogenesis in the plant or spread in plant population
5. Prevention of disease depends on management of primary inoculum
6. Integration of different approaches of disease management is always recommended
General principles of plant disease management
1. Avoidance: Avoiding disease by planting at times when, or in areas where, inoculum
is ineffective due to environmental conditions, or is rare or absent
2. Exclusion of inoculum: Preventing the inoculum from entering or establishing in the
field or area where it does not exist
3. Eradication: Reducing, inactivating, eliminating or destroying inoculum at the source,
either from a region or from an individual plant in which it is already established
4. Protection: Preventing infection by creating a chemical toxic barrier between the plant
surface and the pathogen
5. Disease resistance (Immunization): Preventing infection or reducing effect of
infection by managing the host through improvement of resistance in it by genetic
manipulation or by chemical therapy.
I. Avoidance of the pathogen: These methods aim at avoiding the contact between the
pathogen and susceptible stage of the crop. This is achieved by
a. Proper selection of geographical area
b. Proper selection of the field
c. Adjusting time of sowing
d. Disease escaping varieties
e. Proper selection of seed and planting material
a) Proper selection of geographical area: Many fungal and bacterial diseases are more
severe in wet areas than in dry areas. Cultivation of bajra in wet areas is not profitable
due to the diseases, smut (Tolyposporium penicillariae) and ergot (Claviceps
microcephala).
b) Proper selection of the field: Proper selection of field will help in the management of
many diseases, especially the soil borne diseases. Raising of a particular crop year after
year in the same field makes the soil sick, where disease incidence and severity may be
more.
Ex: Wilt of redgram, late blight of potato (Phytophthora infestans), green ear of bajra
(Sclerospora graminicola), etc.
39
c) Time of sowing: Generally pathogens are able to infect the susceptible plants under
certain environmental conditions. Alteration of date of sowing can help in avoidance of
favourable conditions for pathogen.
Ex: Rhizoctonia root rot of redgram is more severe in the crop sown immediately after the
rains. Delayed sowing will help in reducing the incidence of disease.
Ex: Infection of black stem rust of wheat (Puccinia graminis tritici) is more in late
sowing, hence, early sowing helps in reduction of stem rust incidence.
d) Disease escaping varieties: Certain varieties of crops escape the disease damage
because of their growth characteristics. Ex: Early maturing varieties of wheat or pea
escape the damage due to Puccinia graminis tritici and Erysiphe polygoni, respectively.
e) Proper selection of seed and planting material: Selection of seed and seedling
material from healthy sources will effectively manage the diseases such as loose smut of
wheat (Ustilago nuda tritici), bunchy top of banana (Banana virus-1), Panama wilt of
banana (Fusarium oxysporum f.sp. cubense) and whip smut of sugarcane (Ustilago
scitaminae). Potato seed certification or tuber indexing is followed for obtaining virus
free seed tubers. Citrus bud wood certification programme will help in obtaining virus
free planting material.
II. Exclusion of the pathogen: These measures aim at preventing the inoculum from
entering or establishing in the field or area where it does not exist. Different methods of
exclusion are seed treatment, seed inspection & certification, and plant quarantine
regulation.
a) Seed inspection and certification: Crops grown for seed purpose are inspected
periodically for the presence of diseases that are disseminated by seed. Necessary
precautions are to be taken to remove the diseased plants in early stages, and then the
crop is certified as disease free. This practice will help in the prevention of inter and intra
regional spread of seed borne diseases.
b) Plant quarantine regulation: Plant quarantine is defined as “ a legal restriction on the
movement of agricultural commodities for the purpose of exclusion, prevention or
delaying the spread of the plant pests and diseases in uninfected areas”.
Plant quarantine laws were first enacted in France (1660), followed by Denmark (1903)
and USA (1912). These rules were aimed at the rapid destruction or eradication of
barbery bush which is an alternate host of Puccinia graminis tritici.
In India, plant quarantine rules and regulations were issued under Destructive Insects
and Pests Act (DIPA) in 1914. In India, 16 plant quarantine stations are in operation by
the “Directorate of plant protection and quarantine” under the ministry of food and
agriculture, government of India.
Plant quarantine measures are of 3 types.
1. Domestic quarantine: Rules and regulations issued prohibiting the movement of
insects and diseases and their hosts from one state to another state in India is called
domestic quarantine. Domestic quarantine in India exists for two pests (Rooted scale and
Sanjose scale) and three diseases (Bunchy top of banana, banana mosaic and wart of
potato).
Bunchy top of banana: It is present in Kerala, Assam, Bihar, West Begal and Orissa.
Transport of any part of Musa species excluding the fruit is prohibited from these states
to other states in India.
Banana mosaic: It is present in Maharashtra and Gujarat. Transport of any part of Musa
species excluding the fruit is prohibited from these states to other states in India.
Wart of potato: It is endemic in Darjeeling area of West Bengal, therefore seed tubers are
not to be imported from West Bengal to other states.
2. Foreign quarantine: Rules and regulations issued prohibiting the import of plants,
plant materials, insects and fungi into India from foreign countries by air, sea and land.
Foreign quarantine rules may be general or specific. General rules aim at prevention of
introduction of pests and diseases into a country, where as the specific rules aim at
40
specific diseases and insect pests. The plant materials are to be imported only through the
prescribed ports of entry.
1. Airports: Bombay (Santacruz), Calcutta (Dum Dum), Madras (Meenambakam), New
delhi (Palam, Safdarjung) and Tiruchurapally.
2. Sea ports: Bombay, Calcutta, Vishakapatnam, Trivandrum, Madras, Tuticorin, Cochin
and Dhanushkoti.
3. Land frontiers: Hussainiwala (Ferozpur district of Punjab), Kharla (Amritsar district
of Punjab) and Sukhiapokri (Darjeeling district of West Bengal)
3. Total embargoes: Total restriction on import and export of agricultural commodities.
Phytosanitary certificate: It is an official certificate from the country of origin, which
should accompany the consignment without which the material may be refused from
entry.
Plant diseases introduced into India before/after enforcement of plant quarantine laws:
S.No. Disease
Year
Introduced into
From
1
Late blight of potato
1883
India
Europe
2
Coffee rust
1879
India
Srilanka
3
Flag smut of wheat
1906
India
Australia
4
Downy mildew of grapes
1910
India
Europe
5
Bacterial blight of rice
1964
India
Phillippines
6
Rice blast
1918
India (Madras)
South East Asia
7
Downy mildew of maize
1912
India (Madras)
Java
8
Ergot of bajra
1957
India (Bombay)
Africa
9
Panama wilt of banana
1920
India
Panama canal
10
Bunchy top of banana
1940
India
Srilanka
11
Wart of potato
1953
India
Netherlands
12
Golden cyst nematode of 1961
India
Europe
potato
Diseases not entered into India: Swollen shoot of cocoa, leaf blight of rubber and many
viral diseases.
III. Eradication: These methods aim at breaking the infection chain by removing the
foci of infection and starvation of the pathogen (i.e., elimination of the pathogen from the
area by destruction of sources of primary and secondary inoculum). It is achieved by
a) Rouging: Removal of diseased plants or their affected organs from field, which
prevent the dissemination of plant pathogens.
Ex: Loose smut of wheat and barley, whip smut of sugarcane, red rot of sugarcane, ergot
of bajra, yellow vein mosaic of bhendi, khatte disease of cardamom, etc. During 19271935, to eradicate citus canker bacterium in USA, 3 million trees were cut down and
burnt.
b) Eradication of alternate and collateral hosts: Eradication of alternate hosts will help
in management of many plant diseases.
Ex: Barbery eradication programme in France and USA reduced the severity of black
stem rust of wheat
Ex: Eradication of Thalictrum species in USA to manage leaf rust of wheat caused by
Puccinia recondita.
Eradication of collateral hosts, such as Panicum repens, Digitaria marginata will help in
the management of rice blast disease (Pyricularia oryzae)
c) Crop rotation: Continuous cultivation of the same crop in the same field helps in the
perpetuation of the pathogen in the soil. Soils which are saturated by the pathogen are
often referred as sick soils. To reduce the incidence and severity of many soil borne
diseases, crop rotation is adopted. Crop rotation is applicable to only root inhabitants and
facultative saprophytes, and may not work with soil inhabitants.
41
Ex: Panama wilt of banana (long crop rotation), wheat soil borne mosaic (6 yrs) and club
root of cabbage (6-10 yrs), etc.
d) Crop sanitation: Collection and destruction of plant debris from soil will help in the
management of soil borne facultative saprophytes as most of these survive in plant debris.
Collection and destruction of plant debris is an important method to reduce the primary
inoculum.
e) Manures and fertilizers: The deficiency or excess of a nutrient may predispose a plant
to some diseases. Excessive nitrogen application aggravates diseases like stem rot,
bacterial leaf blight and blast of rice. Nitrate form of nitrogen increases many diseases,
whereas, phosphorous and potash application increases the resistance of the host.
Addition of farm yard manure or organic manures such as green manure, 60-100 t/ha,
helps to manage the diseases like cotton wilt, Ganoderma root rot of citrus, coconut, etc.
f) Mixed cropping: Root rot of cotton (Phymatotrichum omnivorum) is reduced when
cotton is grown along with sorghum. Intercropping sorghum in cluster bean reduces the
incidence of root rot and wilt (Rhizoctonia solani)
g) Summer ploughing: Ploughing the soil during summer months expose soil to hot
weather which will eradicate heat sensitive soil borne pathogens.
h) Soil amendments: Application of organic amendments like saw dust, straw, oil cake,
etc., will effectively manage the diseases caused by Pythium, Phytophthora, Verticillium,
Macrophomina, Phymatotrichum and Aphanomyces. Beneficial micro-organisms
increases in soil and helps in suppression of pathogenic microbes.
Ex: Application of lime (2500 Kg/ha) reduces the club root of cabbage by increasing soil
pH to 8.5
Ex: Application of Sulphur (900 Kg/ha) to soil brings the soil pH to 5.2 and reduces the
incidence of common scab of potato (Streptomyces scabies).
ij) Changing time of sowing: Pathogens are able to infect susceptible plants under certain
environmental conditions. Alternation in date of sowing can help avoidance of favourable
conditions for the pathogens.
Ex: Rice blast can be managed by changing planting season from June to
September/October.
j) Seed rate and plant density: Close spacing raises atmospheric humidity and favours
sporulation by many pathogenic fungi. A spacing of 8’X8’ instead of 7’X7’ reduces
sigatoka disease of banana due to better ventilation and reduced humidity. High density
planting in chillies leads to high incidence of damping off in nurseries.
k) Irrigation and drainage: The amount, frequency and method of irrigation may affect
the dissemination of certain plant pathogens. Many pathogens, including, Pseudomonas
solanacearum, X. campestris pv. oryzae and Colletotrichum falcatum are readily
disseminated through irrigation water. High soil moisture favours root knot and other
nematodes and the root rots caused by species of Sclerotium, Rhizoctonia, Pythium,
Phytophthora, Phymatotrichum, etc.
42
LECTURE 14
PHYSICAL METHODS: Physical methods include soil solarization and hot water
treatments.
i. Soil solarization: Soil solarization or slow soil pasteurization is the hydro/thermal soil
heating accomplished by covering moist soil with polyethylene sheets as soil mulch
during summer months for 4-6 weeks. Soil solarization was developed for the first time in
Israel (Egley and Katan) for the management of plant pathogenic pests, diseases and
weeds.
ii. Soil sterilization: Soil can be sterilized in green houses and sometimes in seed beds by
aerated steam or hot water. At about 500C, nematodes, some oomycetous fungi and other
water molds are killed. At about 60 and 720C, most of the plant pathogenic fungi and
bacteria are killed. At about 820C, most weeds, plant pathogenic bacteria and insects are
killed. Heat tolerant weed seeds and some plant viruses, such as TMV are killed at or
near the boiling point (95-1000C).
iii. Hot water or Hot air treatment: Hot water treatment or hat air treatment will
prevent the seed borne and sett borne infectious diseases. Hot water treatment of certain
seeds, bulbs and nursery stock is done to kill many pathogens present in or on the seed
and other propagating materials. Hot water treatment is used for controlling sett borne
diseases of sugarcane [whip smut, grassy shoot and red rot of sugarcane (520C for 30
min)] and loose smut of wheat (520C for 10 min).
Biological methods:
Def: Biological control of plant disease is a condition or practice whereby survival or
activity of a pathogen is reduced through the agency of any other living organism (except
human beings), with the result that there is reduction in incidence of the disease caused
by the pathogen (Garett, 1965).
Def: Biological control is the reduction of inoculum density or disease producing activity
of a pathogen or a parasite in its active or dormant state by one or more organisms
accomplished naturally or through manipulation of the environment of host or antagonist
by mass introduction of one or more antagonists (Baker and Cook, 1974)
Mechanisms of biological control
1. Competition: Most of the biocontrol agents are fast growing and they compete with
plant pathogens for space, organic nutrients and minerals. Most aerobic and facultative
anaerobic micro-organisms respond to low iron stress by producing extracellular, low
molecular weight (500-1000 daltons) iron transport agents, designated as Siderophores,
which selectively make complex with iron (Fe3+) with very high affinity. Siderophore
producing strains are able to utilize Fe3+ - Siderophore complex and restrict the growth of
deleterious micro-organisms mostly at the plant roots. Iron starvation prevents the
germination of spores of fungal pathogens in rhizosphere as well as rhizoplane.
Siderophores produced by
Pseudomonas fluorescens (known as pseudobactins or pyoveridins) helps in the control
of soft rot bacterium, Erwinia caratovora.
2. Antibiosis: Antagonism mediated by specific or non-specific metabolites of microbial
origin, by lytic agents, enzymes, volatile compounds or other toxic substances is known
as antibiosis.
a. Antibiotics: Antibiotics are generally considered to be organic compounds of low
molecular weight produced by microbes. At low concentrations, antibiotics are
deleterious to the growth or metabolic activities of other micro-organisms.
Ex: Gliocladium virens produces gliotoxin that was responsible for the death of
Rhizoctonia solani on potato tubers.
Ex: Colonization of pea seeds by Trichoderma viride resulted in the accumulation of
significant amount of the antibiotic viridin in the seeds, thus controlling Pythium
ultimum.
43
Ex: Some strains of Pseudomonas fluorescens produce a range of compounds, viz., 2,4diacetyl phloroglucinol (DAPG), phenazines, pyocyanin, which have broad spectrum
activity against many plant pathogenic bacteria and fungi
b. Bacteriocins: These are antibiotic like compounds with bactericidal specificity closely
related to the bacteriocin producer. Ex: The control of crown gall (caused by
Agrobacterium tumefaciens) by the related Agrobacterium radiobacter strain K 84 is by
the production of bacteriocin, Agrocin K84.
c. Volatile compounds: Antibiosis mediated by volatile compounds has been observed in
the management of soil borne pathogens, viz., Pythium ultimum, Rhizoctonia solani and
Verticillium dahlia, by Enterobacter cloacae. The volatile fraction responsible for
inhibition was identified as ammonia.
3. Hyperparasitism: Direct parasitism or lysis and death of the pathogen by another
micro-organism when the pathogen is in parasitic phase is known as hyperparasitism.
Ex: T. harzianum parasitize and lyse the mycelia of Rhizoctonia and Sclerotium.
Biocontrol agents for the management of plant pathogens
Biocontrol agent
Pathogen/disease
1. Ampelomyces quisqualis
Powdery mildew fungi
2. Darluca filum, Verticillium lecanii
Rust fungi
3. Pichia gulliermondii
Botrytis, Penicillium
Biocontrol agent
1. Pasteuria penetrans (Bacteria)
2. Paecilomyces lilacinus (Fungus)
Nematode
Juvenile parasite of root knot nematode
Egg parasite of Meloidogyne incognita
Important fungal biocontrol agents:
Most of the species of Trichoderma, viz., T. harzianum, T. viride, T. virens (Gliocladium
virens) are used as biocontrol agents against soil borne diseases, such as, root rots,
seedling rots, collar rots, damping off and wilts caused by the species of Pythium,
Fusarium, Rhizoctonia, Macrophomina, Sclerotium, Verticillium, etc.
Formulations of biocontrol agents available: T. viride (Ecofit, Bioderma in India), G.
virens (GlioGard in USA), T. harzianum (F-Stop in USA) and T. polysporum (BINABT)
Important bacterial biocontrol agents:
1. Pseudomonas fluorescens (Dagger-G against damping off of cotton seedlings in USA)
2. Bacillus subtilis (Kodiak against damping off and soft rot in USA)
3. Agrobacterium radiobacter K-84 (Gallex or Galltrol against crown gall of stone fruits
caused by Agrobacterium tumefaciens)
Plant growth promoting Rhizobacteria (PGPR):
Rhizosphere bacteria that favourably affect plant growth and yield of commercially
important crops are designated as plant growth promoting rhizobacteria. The growth
promoting ability of PGPR is due to their ability to produce phytohormones,
Siderophores, Hydrogen cyanide (HCN), chitinases, volatile compounds or antibiotics
which will reduce infection of host through phyto-pathogenic micro-organisms.
Many bacterial species, viz., Bacillus subtilis, Pseudomonas fluorescens, etc., are usually
used for the management of plant pathogenic microbes. Bacillus has ecological
advantages as it produces endospores that are tolerant to extreme environmental
conditions. Pseudomonas fluorescens have been extensively used to manage soil borne
plant pathogenic fungi due to their ability to use many carbon sources that exude from the
roots and to compete with microflora by the production of antibiotics, HCN and
Siderophores that suppress plant root pathogens.
44
LECTURE 15
PROTECTION: Use of chemicals for the control of plant diseases is generally referred
to as protection or therapy.
Protection: The prevention of the pathogen from entering the host or checking the further
development in already infected plants by the application of chemicals is called
protection and the chemicals used are called protectants.
Therapy means cure of a disease, in which fungicide is applied after the pathogen is in
contact with the host. Chemicals used are called therapeutants.
Fungicide: Any agent (chemical) that kills the fungus
Fungistat: Some chemicals which do not kill fungi, but simply inhibit the fungus growth
temporarily.
Antisporulant: The chemical which inhibits spore production without affecting
vegetative growth of the fungus.
Fungicides are classified into three categories: Protectants, eradicants and therapeutants.
1. Protectants: These are the chemicals which are effective only when used before
infection (prophylactic in behavior). Contact fungicides which kill the pathogen present
on the host surface when it comes in contact with the host are called protectants. These
are applied to seeds, plant surfaces or soil. These are non-systemic in action (i.e, they
cannot penetrate plant tissues). Ex: Zineb,sulphur, captan, Thiram, etc.
2. Eradicants: Those chemicals which eradicate the dormant or active pathogen from the
host. They can remain on/in the host for some time. Ex: Lime sulphur, Dodine.
3. Therapeutants: These are the agents that inhibit the development of a disease
syndrome in a plant when applied after infection by a pathogen. Therapy can be by
physical means (solar and hot water treatment) and chemical means (by use of systemic
fungicides, i.e., chemotherapy).
CLASSIFICATION OF FUNGICIDES BASED ON CHEMICAL NATURE
Many fungicides have been developed for purpose of managing crop diseases which may
be used as sprays, dusts, paints, pastes, fumigants, etc. The discovery of Bordeaux
mixture in 1882 by Professor Millardet, University of Bordeaux, France led to the
development of fungicides. Major group of fungicides used include salts of toxic metals
and organic acids, organic compounds of sulphur and mercury, quinones and heterocyclic
nitrogenous compounds. Copper, mercury, zinc, tin and nickel are some of the metals
used as base for inorganic and organic fungicides. The non metal substances include,
sulphur, chlorine, phosphorous etc. The fungicides have been classified based on their
chemical nature as follows
COPPER FUNGICIDES: Copper fungicides can be classified as preparatory and
proprietory copper compounds.
PREPARATORY COPPER FUNGICIDES
Common name
1. Bordeaux
mixture
2.Bordeaux paste
Chemical composition
It is prepared by suspending 5
Kg of copper sulphate and 5
Kg of lime in 500 liters of
water (1%)
It is prepared by mixing 1 Kg
of copper sulphate and 1 Kg
of lime in 10 liters of water
Diseases managed
Downy mildew of grapes, Coffee rust,
Tikka leaf spot of groundnut, citrus
canker, citrus scab, etc.
It is a wound dressing fungicide and can
be applied to the pruned parts of the host
plants such as fruit crops and ornamentals.
Ex: Citrus gummosis, Stem bleeding of
coconut, Bud rot of coconut, etc.
45
3.Burgundy
mixture
Sodium carbonate is used in Downy mildew of grapes, Coffee rust,
place of lime. It is prepared Tikka leaf spot of groundnut, citrus
by mixing 1 Kg of copper canker, citrus scab
sulphate and 1 Kg of sodium
carbonate in 100 liters of
water
4.Cheshunt
compound
It is a compound prepared by
mixing 2 parts of copper
sulphate and 11 parts of
ammonium carbonate
5.Chaubattia
paste
It is used for soil drenching only.
Sclerotial wilt diseases of chilli, tomato
and groundnut. Fusarial wilt diseases.
Damping-off diseases of solanaceous
crops.
It is a compound prepared by Pink disease of citrus, stem canker and
mixing 800g of copper collar rot of apple and pears
sulphate and 800g of red lead
in 1 liter of lanolin or linseed
oil
Proprietary copper fungicides or Fixed or insoluble copper fungicides: In the fixed or
insoluble copper compounds, the copper ion is less soluble than in Bordeaux mixture. So,
these are less phytotoxic than Bordeaux mixture but are effective as fungicides.
Common name
Trade name
1.
Copper
oxy Blitox-50,
chloride
Blue copper50, Cupramar50
2. Cuprous oxide
Dosage
0.3 to 0.5% for foliar
application,
25 to 35 Kg/ha for
dusting
Fungimar and 0.3% for foliar spray
Perenox
3. Copper hydroxide Kocide
0.3% for foliar spray
Disease managed
Anthracnose of grapevine,
Tikka
leaf
spot
of
groundnut, Sigatoka leaf
spot of banana, citrus
canker, black arm of cotton
Anthracnose of grapevine,
Tikka
leaf
spot
of
groundnut, Sigatoka leaf
spot of banana, citrus
canker, black arm of cotton
Blister blight of tea, False
smut of rice, Tikka leaf spot
of groundnut
SULPHUR FUNGICIDES
Sulphur is probably the oldest chemical used in plant disease management for the control
of powdery mildews and can be classified as inorganic sulphur and organic sulphur.
Inorganic sulphur fungicides include lime sulphur and elemental sulphur fungicides.
Organic sulphur fungicides, also called as carbamate fungicides, are the derivatives of
dithiocarbamic acid.
INORGANIC SULPHUR FUNGICIDES
Common name
Trade name
Preparatory sulphur compounds
1. Lime sulphur
It is prepared by
mixing 20 Kg of
rock lime and 15 Kg
of sulphur in 500
liters of water
2. Sulphur dust
Kolo dust, Mico999
3. Wettable sulphur
Sulfex,
Cosan
Dosage
Disease managed
10-15 liters in 500 Powdery mildew of
liters of water
apple, Apple scab,
bean rust
4-5g/Kg seed for
ST, 10-30 Kg/ha for
dusting on crops,
100 Kg /ha for soil
application
in
tobacco, 500 Kg/ha
for
furrow
application in potato
Thiovit, 0.2-0.4 % for foliar
spray
Common scab of
potato, Grain smut
of jowar, Powdery
mildew of tobacco,
chilli, rose, mango,
grapes, etc.
Powdery mildews of
various crops
46
ORGANIC SULPHUR COMPOUNDS
Organic sulphur compounds are derived from dithiocarbamic acid and are widely used as
spray fungicides. In 1931, Tisdale and Williams were the first to describe the fungicidal
nature of Dithiocarbamates. Dithiocarbamates can be categorized into two groups, viz.,
dialkyl dithiocarbamates (ziram, ferbam and thiram) and monoalkyl dithiocarbamates
(nabam, zineb, vapam and maneb).
Common name
Trade name
Dialkyl Dithiocarbamates
1. Ziram
Ziride, Hexazir,
Milbam, Zerlate
2. Ferbam
3. Thiram
Coromet, Ferbam,
Fermate,
Fermocide,
Hexaferb, Karbam
Black
Arasan, Hexathir,
Tersan, Thiram,
Thiride
Dosage
Diseases managed
0.15 to 0.25%
for foliar spray
Anthracnose of pulses,
tomato, beans, tobacco, etc.,
bean rust
Fungal pathogens of fruits
and vegetables, leaf curl of
peaches, apple scab, downy
mildew of tobacco
0.15 to 0.25%
for foliar spray
0.15 to 0.2% as
foliar spray, 0.20.3% as dry seed
treatment,
1525Kg/ha as soil
application
Soil borne diseases caused
by Pythium, Rhizoctonia
solani, Fusarium, etc. Rust
of ornamental crops, Scab
on pears and Botrytis spp.
on lettuce
Monoalkyl dithyiocarbamates
1. Nabam
Chembam,
0.2% as foliar Used as foliar spray against
Dithane
D-14, spray
leaf spot diseases of fruits
Dithane A-40 and
and vegetables. Also used
Parzate liquid
against soil borne
pathogens, Fusarium,
Pythium and Phytophthora
2. Zineb
Dithane
Z-78,
Hexathana,
Lanocol
and
Parzate
3.Vapam
or Chem-vape,
Metham sodium vapam, vitafume,
VPM
4. Maneb
0.1 to 0.3% for Chilli die-back and fruit rot,
foliar application Apple scab, Maize leaf
blight, early blight of potato
1.5 to 2.5 liters Fungicide with fungicidal,
nematicidal and insecticidal
per 10 m2 area
properties.
Soil
fungal
pathogens like Fusarium,
Puthium, Sclerotium and
Rhizoctonia.
Dithane
M22, 0.2% to 0.3% as Early and late blight of
Manzate
and foliar application potato and tomato, rust
MEB.
diseases of field and fruit
Mancozeb (78%
crops
Maneb + 2% zinc
ion): Dithane M
45, Indofil M 45
HETROCYCLIC NITROGENOUS COMPOUNDS
The group of heterogeneous fungicides includes some of the best fungicides like captan,
folpet, captafol, vinclozoline and Iprodione. Captan, folpet and captafol belong to
dicarboximides and are known as pthalamide fungicides. The new members of
dicarboximide group are Iprodione, vinclozolin, etc.
47
Common name
1.Captan
(Kittleson’s killer)
Trade name
Captan
50W,
Captan 75 W, Esso
fungicide, Orthocide
406,
Hexacap,
Vancide 89
Dosage
0.2 to 0.3% for dry
seed treatment, 0.2
to 0.3% for foliar
spray, 25 to 30
Kg/ha for furrow
application
2. Folpet
Phaltan
0.1 to 0.2%
spraying
3.Captafol
Difosan, Difolaton, 0.15 to 0.2% for
Sanspor, Foltaf
spraying, 0.25% for
seed
treatment,
0.15%
for
soil
drenching
4. Iprodione
Rovral, Glycophene
5. Vinclozolin
Ornalin,
Vorlan
for
0.1 to 0.2% for
foliar application
Ronilan, 0.1 to 0.2% for
foliar application
Diseases managed
Onion smut, Chilli
die-back and fruit
rot, Damping off of
beans, chilli and
tomato, seed rots
and seedling blights
of maize
Apple scab, tobacco
brown spot, rose
black spot
Sorghum
anthracnose, cotton
seedling diseases,
seed
rot
and
seedling diseases of
rice, downy mildew
of crucifers, apple
scab
Diseases caused by
Botrytis, Monilinia,
Alternaria,
Sclerotinia,
Helminthosporium
and Rhizoctonia
Effective
against
sclerotia
forming
fungi like Botrytis,
Monilinia
and
Sclerotinia
MISCELLANEOUS FUNGICIDES
Common name
Trade name
Dosage
1. Chlorothalonil
Bravo,
Daconil, 0.2 to 0.3% for
Kavach,
Thermil, foliar application
Exotherm,
Safegaurd
2. Dinocap
Karathane,
0.1 to 0.2%
Arathane, Capryl, spraying
Mildex, Mildont and
crotothane
Diseases managed
A broad spectrum
contact
fungicide
often
used
in
greenhouses
for
control of Botrytis
on ornamentals and
for several molds
and
blights
of
tomato. Also used
for the control of
sigatoka leaf spot of
banana,
onion
purple blotch, tikka
leaf spot and rust of
groundnut
for It
is
a
good
acaricide
and
contact
fungicide
and
it
controls
powdery mildews of
fruits
and
ornamentals
effectively. This can
be safely used on
sulphur
sensitive
crops like cucurbits
and apple varieties
for
control
of
powdery mildews
48
3. Dodine
Cyprex, Melprex, 0.075% for spraying
Guanidol and Syllit
Apple scab, black
spot of roses and
cherry leaf spot
SYSTEMIC FUNGICIDES
The systemic fungicides were first introduced by Von Schelming and Marshall Kulka in
1966. The discovery of Oxathiin fungicides was soon followed by confirmation of
systemic activity of pyrimidines and benzimidazoles. A systemic fungicide is capable of
managing a pathogen remote from the point of application. On the basis of chemical
nature these fungicides are classified as follows
Common name
ACYLALANINES
1. Metalaxyl
2. Benalaxyl
Trade name
Dosage
Diseases managed
Ridomil
25
% WP, Apron 35
SD,
Subdue,
Ridomil MZ-72WP
It is highly effective
against
Pythium,
Phytophthora
and
many downy mildew
fungi
Galben 25%
and 5% G
Blue mold of tobacco,
late blight of potato
and tomato, downy
mildew of grapevine
AROMATIC HYDROCARBONS
1. Chloroneb
Demosan
BENZIMIDAZOLES
1. Carbendazim
Bavistin
MBC,
60WP,
Zoom
3-6 g/Kg seed for
seed treatment, 1
to 1.5 Kg a.i/ha
for
soil
application, 0.1
to 0.2% for foliar
spray
WP 0.1 to 0.2% for
foliar spray, 1 to
1.5 Kg a.i/ha for
soil application
0.2% for
treatment
seed Seedling diseases of
cotton , peanut, peas
and cucurbits caused
by species of Pythium,
Phytophthora,
Rhizoctonia
and
Sclerotium
50WP, 0.1% for foliar
Derosol spray, 0.1% for
Agrozim, soil
drench,
0.25% for ST,
500-1000ppm for
post-harvest dip
of fruits
2. Benomyl
Benlate 50WP
0.1 to 0.2% for
ST, 50-60g/100
L for foliar spray,
50-200ppm for
soil drenching,
12-45 Kg a.i/ha
for
soil
broadcast, 100500 ppm for post
harvest fruit dip
3. Thiabendazole
Mertect
60WP, 0.2 to 0.3% for
Mycozol, Arbotect, spraying, 1000
Tecto and Storite
ppm for fruit dip
Effectively
controls
anthracnose, powdery
mildews and rusts
caused by various
fungi. It is also used as
a soil drench against
wilt diseases and for
post harvest treatment
of fruits
Effective
against
powdery mildews of
cucurbits, cereals and
legumes. It is highly
effective
against
diseases caused by the
species of Rhizoctonia,
Theilaviopsis
and
Cephalosporium.
Benomyl has no effect
against Oomycetes and
some dark coloured
fungi
such
as
Alternaria
and
Helminthosporium
Blue and green molds
of citrus, loose smut of
wheat, Tikka leaf spot
of groundnut
49
ALIPHATICS
1. Prothiocarb
Previcur, Dynone
5.6 Kg a.i/ha for Highly active against
soil application
soil borne Oomycetes
like
Pythium
and
Phytophthora
2. Propamocarb
Previcur-N,
3.4 and 4.8 Kg Effective
against
Dynone-N, Prevex, a.i/ha for soil against
soil
borne
Benol
application
Oomycetes
like
Pythium
and
Phytophthora
OXATHINS or CARBOXIMIDES
1. Carboxin
Vitavax
75WP, 0.15 to 0.2% for Highly
effective
Vitaflow
seed treatment, against smut diseases.
0.5%
for Commonly used for the
spraying
control of loose smut
of wheat, onion smut,
grain smut of sorghum.
As a soil drench it is
used for the control of
diseases caused by
Rhizoctonia solani and
Macrophomina
phaseolina.
2. Oxycarboxin
Plantavax 75 WP, 0.1 to 0.2% for Highly
effective
Plantavax
20EC, foliar spray, 0.2 against rust diseases.
Plantavax 5% liquid to 0.5% for ST
Commonly used for the
control of rusts of
wheat,
sorghum,
safflower, legumes, etc.
IMIDAZOLES
1. Imazalil
Fungaflor, Bromazil 0.1 % as post Blue and green molds
and Nuzone
harvest dip
of citrus
2. Fanapanil
Sistane 25 EC
0.05%
foliar Spot blotch of barley,
spray
loose and covered smut
of barley
MORPHOLINES
1. Tridemorph
Calixin
75EC, 0.1% for foliar Powdery mildew of
Bardew, Beacon
spray
cereals, vegetables and
ornamentals. Rusts of
pulses, groundnut and
coffee, Sigatoka leaf
spot of banana, pink
disease of rubber,
Ganoderma root rot &
wilt
ORGANOPHOSPHATES
1. Iprobenphos
Kitazin
48EC, 30-45 Kg of Fungicide
with
Kitazin
17G, granules/ha,
insecticidal properties.
Kitazin 2% D
1 to 1.5 liters of Highly specific against
48% EC in 1000 rice blast, stem rot and
ml of water for sheath blight of rice
foliar spray
2. Ediphenphos
Hinosan 30 and 400 to 500 ppm Highly specific against
50% EC, Hinosan for spraying, 30 rice blast, stem rot and
2%D
to 40 Kg/ha
sheath blight of rice
ALKYL PHOSPHONATES
1. Fosetyl-Al or Aliette 80WP
0.15% for foliar Ambimobile fungicide.
Aluminium Tris
spray, 0.2% for Specific
against
soil drench
Oomycetes fungi
50
PYRIMIDINES
1. Fenarimol
THIOPHANATES
1. Thiophanate
2.Thiophanate
methyl
TRIAZOLES
1. Triadimefon
2. Tricyclazole
3. Bitertanol
Rubigan 50% WP, 2g/Kg seed
12%EC
ST, 20 to
ml/100 liters
water
spraying
Powdery mildew of
cucurbits,
apple,
mango, roses, grapes
and ornamental crops
Topsin
50WP, 0.1 to 0.2% for Powdery mildew of
Cercobin 50WP
spraying
cuurbits and apple,
club root of crucifers,
rice blast
Topsin M 70WP, 0.1%
for Blast and sheath blight
Cercobin M 70WP
spraying
of rice, sigatoka leaf
spot
of
banana,
powdery mildew of
beans, chilli, peas and
cucurbits
Bayleton, Amiral
Beam 75WP, Baan
75WP,
Trooper
75WP
Baycor and Sibutol
4. Hexaconazole
Contaf
Anvil
5. Propiconazole
Tilt,
25%
Desmel
6. Myclobutanil
Systhane 10WP
STROBILURINS
1. Azoxystrobin
Amistar, Quadris
2.Kresoxim methyl
as
40
of
for
Ergon,
Stroby
5%EC,
EC,
0.1 to 0.2% for
spraying, 0.1%
for
seed
treatment
Highly
effective
against
powdery
mildews and rusts of
several crops. Effective
against diseases caused
by species of Erysiphe,
Sphaerotheca,
Puccinia, Uromyces,
Phakopsora, Hemileia
and Gymnosporangium
2g/Kg seed for Highly
effective
ST, 0.06% for against blast of rice
spraying
0.05 to 0.1% for Powdery mildews and
foliar spray
rusts of various crops,
apple scab, Monilinia
on fruit crops, late leaf
spot of groundnut and
sigatoka leaf spot of
banana
0.2%
for Sheath blight of rice,
spraying
powdery mildew and
rust of apple, rust and
tikka leaf spot of
groundnut
0.1% for foliar Sheath blight of rice,
application
Sigatoka leaf spot of
banana, brown rust of
wheat
0.1 to 0.2% for Apple scab, cedar
spraying
apple rust and powdery
mildew of apple
0.1%
spraying
Discus, 0.1%
spraying
for Broad
spectrum
fungicide
for Commonly used for
control of ornamental
diseases
CLASSIFICATION OF FUNGICIDES BASED ON METHOD OF APPLICATION
The fungicides can also be classified based on the nature of their use in managing the
diseases.
51
1. Seed protectants: Ex. Captan, thiram, carbendazim, carboxin etc.
2. Soil fungicides (preplant): Ex. Bordeaux mixture, copper oxy chloride, Chloropicrin,
Formaldehyde, Vapam, etc.
3. Soil fungicides: Ex. Bordeaux mixture, copper oxy chloride, Captan, PCNB, thiram
etc.
4. Foliage and blossom: Ex. Capton, ferbam, zineb, mancozeb, chlorothalonil etc.
5. Fruit protectants: Eg. Captan, maneb, carbendazim, mancozeb etc.
6. Eradicants: EX. Lime sulphur
7. Tree wound dressers: Ex. Boreaux paste, chaubattia paste, etc.
8. General purpose sprays and dust formulations.
HOST PLANT RESISTANCE (IMMUNIZATION)
Disease resistance: It is the ability of a plant to overcome completely or in some degree
the effect of a pathogen or damaging factor.
Susceptibility: The inability of a plant to resist the effect of a pathogen or other
damaging factor.
Advantages of resistant varieties:
1. Resistant varieties can be the most simple, practical, effective and economical method
of plant disease management.
2. They not only ensure protection against plant diseases but also save the time, energy
and money spent on other measures of control
3. Resistant varieties, if evolved can be the only practical method of control of diseases
such as wilts, viral diseases, rusts, etc.
4. They are non-toxic to human beings, animals and wild life and do not pollute the
environment
5. They are effective only against the target organisms, whereas, chemical methods are
not only effective against target organisms but also effective against non-target
organisms.
6. The resistance gene, once introduced, is inherited and therefore permanent at no extra
cost.
Disadvantages:
1. Breeding of resistant varieties is a slow and expensive process
2. Resistance of the cultivar may be broken down with the evolution of the pathogen
Types of resistance:
1. Vertical resistance: When a variety is more resistant to some races of the pathogen
than others, the resistance is called vertical resistance (race-specific resistance, qualitative
resistance, discriminatory resistance). Vertical resistance is usually governed by single
gene and is unstable.
2. Horizontal resistance: When the resistance is uniformly spread against all the races of
a pathogen, then it is called horizontal/generalized/non-specific/field/qualitative
resistance. Horizontal resistance is usually governed by several genes and is more stable.
3. Monogenic resistance: When the defense mechanism is controlled by a single gene
pair, it is called monogenic resistance.
4. Oligogenic resistance: when the defense mechanism is governed by a few gene pairs,
it is called oligogenic resistance.
5. Polygenic resistance: When the defense mechanism is controlled by many genes or
more groups of supplementary genes, it is called polygenic resistance.
Cross protection: The phenomenon in which plant tissues infected with mild strain of a
virus are protected from infection by other severe strains of the same virus. This strategy
is used in the management of severe strains of Citrus Tristeza virus
52
INTEGRATED PLANT DISEASE MANAGEMENT (IPDM)
IPDM involve management systems which utilize compatible combinations of all the
available techniques to keep the pathogen population below the economic threshold level
(ETL) which would not result in economically unacceptable damage to the crop. IPDM is
based on five principles of plant disease management and integrates multidisciplinary
approaches for the management of plant diseases.
Main components of IPDM:
1. Cultural practices
2. Regulatory measures (quarantine)
3. Chemical methods
4. Biological methods
5. Physical methods
6. Genetic engineering
Main strategies of IPDM:
1. Need based application of pesticides
2. Encouragement and enhancement of biocontrol agents
3. Use of resistant or tolerant cultivars of plants
4. Modification of cultural practices
5. Use of any other strategies that interrupts host-pathogen interactions
Advantages of IPDM:
1. Avoids chemical pollution of soil, water, air and food products
2. Avoids development of resistance in the plant pathogens against fungicides
3. It is an eco-friendly strategy for management of plant diseases
4. It is an economically feasible approach
5. It is a multipronged strategy for efficient management of plant diseases
Therefore, IPDM utilizes all suitable strategies in a compatible manner to reduce and
maintain pathogen populations at levels below those causing economic losses.
Rice diseases and IPDM:
Fungal diseases
1. Blast: Foliar disease and the pathogen survives on collateral hosts
2. Brown spot of rice – Seed borne and a foliar disease
3. Sheath rot, sheath blight, foot rot and stem rot – Soil borne diseases
4. False smut – seed borne disease
Bacterial diseases: Bacterial leaf blight and bacterial leaf streak – Seed borne and
survives on collateral hosts and weeds
Viral or Phytoplasmal diseases – Rice tungro virus, Rice yellow dwarf – Survives on
weeds and dissemination is by insect vectors
IPDM strategy in rice:
1. Selection of healthy seed
2. Selection of resistant cultivars
3. Removal and destruction of collateral hosts
4. Balanced fertilization
5. Rouging of diseased plants
6. Seed treatment with carbendazim or tricyclazole at 2g/Kg seed
7. Need based foliar application of carbendazim@0.1% or Tricyclazole@0.06% for the
management of blast.
8. Need based foliar application of validamycin for the management of sheath blight and
sheath rot.
9. Soil application of carbofuran granules or foliar spray of any systemic fungicide is
followed to manage insect vectors, thereby decreasing the spread of viral diseases.
Sugarcane diseases and IPDM
1. Red rot – sett borne disease which spreads through irrigation water
2. Whip smut - sett borne and disseminate through wind borne sporidia
53
3. Pine apple disease, sett rot – Sett borne disease
4. Grassy shoot – Vector borne Phytoplasmal disease
5. Ratoon stunting – Sett borne (Clavibacter xyli)
6. Sugarcane mosaic – Survives on weeds and disseminated by insect vectors
IPDM in sugarcane:
1. Collection and destruction of infected crop debris
2. Hot water treatment of setts (520C for 30 min)
3. Hot air treatment of setts (540C for 2-3 hrs)
4. Balanced irrigation and fertilization
5. Avoid selection of seed material from Ratoon crop
6. Need based spray of systemic insecticides to minimize the spread of viral and
Phytoplasmal diseases
7. Selection of disease resistant or tolerant cultivars
54
LECTURE 16
Biotechnology: It is defined as genetic modification and manipulation of living
organisms through the novel technologies such as tissue culture and genetic engineering
resulting in production of improved or new organisms that can be used in variety of ways.
APPLICATION OF BIOTECHNOLOGY IN PLANT DISEASE MANAGEMENT:
1. Diagnosis of plant diseases
a) Diagnostic kits helps in identification of plant diseases, viz., bacterial canker of
tomato, soybean root rot, viral diseases of potato, etc., at an early stage of development
and helps in devising suitable management practices.
b) Polymerase Chain Reaction (PCR): Detection of very small amount of pathogen in a
sample by amplifying the pathogen sequences to a detectable level. PCR is especially
used in plant quarantine.
2. Strain improvement of biocontrol agents: It has the following advantages
a) Expanding the range of target species
b) Restricting the range of non-target species
c) To improve the survival ability or rhizosphere competence
d) Expanding the bio-agents environmental range beyond its congenial habitat
e) Development of fungicide tolerant strains
3. Transgenics for plant disease management
a) Coat protein mediated resistance for papaya ring spot virus in Hawaii islands
b) Cloning of resistance genes, viz., Xa 21, bacterial blight resistance gene isolated from
African rice, Oryza longistaminata was introduced into cultivable rice, Oryza sativa
4. Determination of biochemical nature and the signals involved in plants reaction to
pathogen invasion and disease development. Ex: Host-pathogen interaction has been
studied in rice blast disease incited by Magnaporthe grisea.
5. Manipulation of resistance of host by expression of PR-proteins, antifungal peptides,
etc. Ex: Expression of multiple PR-proteins (Chitinases and β-1,3 glucanases) in rice
enhanced disease resistance to rice sheath blight pathogen, Rhizoctonia solani.
PLANT TISSUE CULTURE: In vitro culture of plant cells, tissues as well as organs.
Totipotency is the ability of a plant cell to perform all the functions of development
which are characteristic of zygote, i.e., its ability to develop in to a complete plant.
IMPORTANT TISSUE CULTURE TECHNIQUES OF IMPORTANCE TO
PLANT PATHOLOGY:
1. Meristem tip culture
2. Protoplast culture
A. Production of virus free plants through plant tissue culture:
Meristem tip culture: Cultivation of axillary or apical meristems, particularly of shoot
apical meristem, is known as meristem culture.
1. Explant: the explant must consist of the meristematic dome of cells together with
atleast one leaf primordial. Meristem tips varying in size from 0.1 to 2.0 mm in diameter
(usually 0.3-1.5 mm) can be used for meristem tip culture. The infected parent plant or
organ of the plant from which explant is excised is generally subjected to thermotherapy
in a temperature controlled cabinet at 300C to 400C for six to twelve weeks to inactivate
the virus.
2. Culture initiation on suitable medium: In general Murashige and Skoog medium has
been found satisfactory for most plant species. But for some species, a much lower salt
concentration may be adequate or even necessary since the high salt concentration of MS
medium may be deleterious or even toxic. Culture initiation consists of surface
sterilization of explants and establishing them in vitro on culture medium. Culture
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initiation often involves anti-metabolite chemicals such as ribavirin (virazole) in the
tissue culture medium.
3. Shoot multiplication: After 2-3 weeks, the cultures are transferred to a shoot
multiplication medium designed to promote axillary branching. This medium generally
contains cytokinins, either alone or in combination with an auxin. Higher concentration
of cytokinins induces adventitious buds. During culture initiation and shoot multiplication
phases, the cultures are generally kept at 250C.
4. Rooting of shoots: In general, the rooting medium has low salt (1/2 or even ¼ salts of
MS medium) and reduced sugar levels. But in most species, 0.1-1 mg/l Naphthalene
Acetic Acid (NAA) or Indole-3-Butyric acid (IBA) is required for rooting. Rooting takes
about 10-15 days depending on species.
5. Transfer of plantlets to soil: Rooted shoots are removed from the medium, agar
sticking to roots is washed with tap water, and they are transplanted into plastic cups
containing a suitable potting mix. Plants are kept in high (>90%) humidity and initially
low light intensities. The humidity is generally decreased to the ambient level after about
7-15 days, and the light intensity is increased. The plants are finally exposed to
greenhouse conditions (hardening).
6. Indexing, clone selection and stock maintenance: Virus indexing is done several
times during first year and the virus free plantlet is used as a nuclear stock material for
commercial multiplication. Virus indexing is generally made by Enzyme Linked
Immuno-Sorbent Assay (ELISA) or Immuno Sorbent Electron Microscopy (ISEM).
B. Protoplast culture: Fungal protoplasts are important tools in physiological and
genetic research. Interspecific, intraspecific and intrageneric hybridization could be done
by this technique for strain improvement of biocontrol agents to enhance the biocontrol
potential for the management of pathogenic fungi. Isolation and self-fusion of protoplasts
were achieved in Trichoderma harzianum and T. viride.
Steps in protoplast fusion:
1. Isolation of protoplasts is achieved by treating cells with a suitable mixture of cell wall
degrading enzymes.
2. The pH of enzyme solution is adjusted between 4.7 and 6.0 and temperature is kept
around 25-300C. The osmotic concentration of enzyme mixture and of subsequent media
is elevated to stabilize the protoplasts and to prevent them from bursting. Usually, 50-100
m mol/l CaCl2 is added to the osmoticum as it improves plasma membrane stability.
3. The protoplasts of different strains are treated with 28-50% Poly Ethylene Glycol
(fusogen) for 15-30 min followed by gradual washing of the protoplasts to remove PEG.
The washing medium may be alkaline and contain high calcium ion concentration (50 m
mol/l). Protoplast fusion occurs during washing step.
4. Selection of hybrid cells and culturing on suitable medium.
Gene cloning/ Recombinant DNA technology / Genetic engineering:
Integration of specific fragment of foreign DNA into a cell through a suitable vector in
such a way that the inserted DNA replicate independently and transferred to progenies as
a result of cell division.
Recombinant DNA molecule is a vector into which the desired DNA fragment has been
inserted to enable its cloning in an appropriate host. Recombinant DNA molecule is
produced by joining together two or more DNA segments usually originated from
different organisms.
Steps in gene cloning:
1. Identification and isolation of the desired gene or DNA fragment to be cloned
(Restriction digestion and electrophoresis)
2. Insertion of the isolated gene in a suitable vector (ligation)
3. Introduction of this vector into a suitable organism or cell called host (transformation)
4. Selection of transformed host cells (selectable markers)
5. Multiplication / integration followed by expression of the introduced gene in the host
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Enzymes involved: Restriction endonucleases, DNA ligases, DNA polymerases, RNA
polymerases and reverse transcriptases.
Vectors used in gene cloning: A vector is a DNA molecule that has the ability to
replicate in an appropriate host cell, and into which the DNA fragment to tbe cloned
(called DNA insert) is integrated for cloning. Ex: Tumor inducing (Ti) plasmid of
Agrobacterium tumefaciens, pBR322, Bacteriophages, cosmid vectors (derived from
phage λ).
Ti plasmid of Agrobacterium tumefaciens:
¾ Ti plasmid is a large conjugative plasmid or megaplasmid of about 200 kb.
¾ Ti plasmid has a T-DNA region (15-24 kb) which is bounded by a pair of 24 bp
repeats. T-DNA carries genes for auxin, cytokinins and opine synthesis which are
responsible for tumor formation (tumorigenesis).
¾ Transfer of T-DNA depends on 35 kb virulence (vir) region of the Ti plasmid. This
region has 7 operons ranging from vir A to vir H (vir A, vir B, vir C, vir D, vir E, vir
G and vir H). The protein products of these genes respond to phenolics to generate a
copy of T-DNA and mediate its transfer into the cell.
¾ The T-DNA when transferred from the Agrobacterium to the plant cell integrates with
the chromosome, and the plant cells which are affected begin to synthesize opines,
auxins and cytokinins.
Auxin biosynthesis
Cytokinin biosynthesis
Opine synthesis
24 bp repeat left border
Vir gene
24 bp repeat right border
Ti Plasmid
tra conjugative transfer
Opine catabolism
Ori (origin of replication)
¾ Opines are tumor specific compounds formed by the condensation of aminoacid,
ketoacid and sugar. The opines (octopine, nopaline, succinamopine or leucinopine)
can be metabolized only by Agrobacteria
¾ The IAA (auxin) and Isopentenyl-AMP (cytokinins) are phytohormones which cause
the proliferation of plant cells and induction of the gall.
¾ Plant wound exudates contain phenolics, which attract Agrobacterium and induce vir
genes. The strong vir gene inducers are syringic acid, ferulic acid, acetosyringone and
sinapinic acid. Only Agrobacterium with Ti plasmid are attracted by these compounds.
¾ The exogenous DNA is inserted into the T-DNA region of the Ti plasmid by
homologous recombination using an intermediate vector system or directly using
binary vectors.
DEVELOPMENT OF DISEASE RESISTANT TRANSGENIC
THROUGH TI PLASMID MEDIATED GENE TRANSFER:
PLANTS
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1. The appropriate gene construct is inserted within the T-DNA of a disarmed Ti plasmid;
either a co-integrate or binary vector is used. The recombinant vector is placed in
Agrobacterium, which is co-cultured with the plant cells or tissues to be transformed for
about 2 days.
2. In case of many plant species, small (a few mm diameter) leaf discs are excised from
surface sterilized leaves and used for co-cultivation. In general the transgene construct
involves a selectable reporter gene (Bacterial neo gene), the presence of which confers
resistance to kanamycin.
3. During the leaf disc-Agrobacterium co-culture, acetosyringone released by plant cells
induces the vir genes which bring about the transfer of recombinant T-DNA into many of
the plant cells. The T-DNA would become integrated into the plant genome, and the
transgene would be expressed. As a result, the transformed plant cells would become
resistant to kanamycin.
4. After 2 days, the leaf discs are transferred onto a regeneration medium containing
appropriate concentrations of kanamycin and carbenicillin. Kanamycin allows only
transformed plant cells to divide and regenerate shoots in about 3-4 weeks, while
carbenicilin kills Agrobacterium cells. The shoots are separated, rooted and finally
transferred into soil.
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